Image Output Control System, Image Processing Device, and Image Processing Method

ABSTRACT

In an image output control system, an image processing device makes image data subjected to a preset series of image processing and supplies processed image data to an image output device to output a resulting image. The image processing device collects a number of pixels among pixels constituting the image to one pixel group, specifies a number of dots to be created in the pixel group, and outputs dot number data representing the specified number of dots to be created in the pixel group to the image output device, which stores multiple priority orders of pixels for dot formation in each pixel group. The image output device receives the output dot number data, selects one priority order among the stored multiple priority orders, determines position of each dot-on pixel in each pixel group, and actually creates a dot at the determined position of each dot-on pixel, so as to output a resulting image. Even when an image includes a large number of pixels, this arrangement ensures quick data supply and thereby high-speed image output.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is continuation of, and claims priority under 35 U.S.C.§120 on, U.S. application Ser. No. 10/550,900, filed on Mar. 19, 2007,the contents of which is incorporated by reference herein. JapanesePatent Application No. 2003-087190 is also incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a technique of making image datasubjected to a preset series of image processing and outputting aprocessed image. More specifically the invention pertains to a techniqueof quickly transferring processed image data to an image output device,so as to attain high-speed image output.

2. Background Art

Image output devices that create dots on various output media, forexample, printing media and liquid crystal screens, to express an imageare widely used as the output device of diverse imaging equipment. Theimage output device divides an image into a number of small elementscalled pixels and creates dots in these pixels. Each pixel takes onlyeither of two states, that is, a dot-on state and a dot-off state. Theimage as a whole may have areas of dense dot formation and areas ofsparse dot formation. Each image is thus expressed by varying the dotformation density. For example, in the case of formation of dots withblack ink on white printing paper, the areas of dense dot formationexpress dark areas, whereas the areas of sparse dot formation expressbright areas. As another example, in the case of formation ofluminescent spots as dots on a liquid crystal screen, the areas of densedot formation express bright areas, whereas the areas of sparse dotformation express dark areas. Adequate regulation of the dot formationdensity enables output of a multi-tone image.

Control data of the dot formation density is obtained by a preset seriesof image processing of object image data, which represents an objectimage to be output. The image-processed data is supplied to the imageoutput device, which then creates dots in pixels specified by thesupplied data. Dots are accordingly created at adequate densities on anoutput medium to express a resulting image.

The higher picture quality and the larger size of output images havebeen demanded for such image output devices. One effective measure tothe better-quality demand divides an image into smaller pixels. Sizereduction of pixels makes dots created in these small pixelsinconspicuous and thereby enhances the picture quality of a resultingimage (see, for example, Japanese Patent Laid-Open Gazette No.2000-115716). The size expansion demand is fulfilled, on the other hand,by increasing the total number of pixels. Size expansion of individualpixels naturally expands the size of an output image, but undesirablylowers the picture quality of the output image. The effective measure tothe size expansion demand thus increases the total number of pixels.

These measures to the better-quality demand and the size expansiondemand of the output image, however, hinder high-speed image output. Letalone the increased number of pixels for the size expansion of theoutput image, the size reduction of individual pixels for the enhancedpicture quality of the output image results in increasing the totalnumber of pixels included in one image. As mentioned above, the imageoutput device receives control data of dot formation and outputs animage according to the received control data. The increase in number ofpixels included in one image thus undesirably extends the time requiredfor data supply and interferes with high-speed image output.

DISCLOSURE OF THE INVENTION

In view of the drawbacks of the prior art techniques, the object of theinvention is to provide a technique of accelerating supply of controldata for dot formation to an image output device and thereby attaininghigh-speed image output.

In order to attain at least part of the above and the other relatedobjects, the present invention is directed to an image output systemhaving an image processing device that makes image data subjected to apreset series of image processing and an image output device thatcreates dots according to a result of the preset series of imageprocessing to output an image.

The image processing device includes: a pixel group setting module thatcollects a predetermined number of plural pixels, among a large numberof pixels constituting the image, to each pixel group; a dot numberspecification module that causes image data of respective pixels in eachpixel group to be represented uniformly by preset representative imagedata and specifies number of dots to be created in each pixel groupaccording to the representative image data; and a number data outputmodule that outputs dot number data representing the specified number ofdots with regard to each pixel group to the image output device.

The image output device includes: a number data receiving module thatreceives the output dot number data with regard to each pixel group; apriority order specification module that specifies a priority order ofpixels for dot formation in each pixel group; a pixel positiondetermination module that determines position of each dot-on pixelincluded in each pixel group, based on the received dot number data andthe specified priority order; and a dot formation module that actuallycreates a dot at the determined position of each dot-on pixel.

There is an image output method corresponding to the image output systemdescribed above. The present invention is thus directed to an imageoutput method that makes image data subjected to a preset series ofimage processing and creates dots according to a result of the presetseries of image processing to output an image.

The image output method includes: a first step of collecting apredetermined number of plural pixels, among a large number of pixelsconstituting the image, to each pixel group; a second step of causingimage data of respective pixels in each pixel group to be representeduniformly by preset representative image data and specifying number ofdots to be created in each pixel group according to the representativeimage data; a third step of specifying a priority order of pixels fordot formation in each pixel group; a fourth step of determining positionof each dot-on pixel included in each pixel group, based on thespecified number of dots and the specified priority order; and a fifthstep of actually creating a dot at the determined position of eachdot-on pixel.

The image output system and the corresponding image output method of theinvention collect a predetermined number of plural pixels to each pixelgroup. Each pixel group may be generated by gathering a plurality ofexisting pixels, by gathering a plurality of smaller pixel divisions ofone identical pixel, which is divided for resolution enhancement of theimage, or by gathering plural pixels generated for size expansion of theimage. The image output system and the image output method specify thenumber of dots to be created in each pixel group, specify the priorityorder of pixels for dot formation in the pixel group, determine theposition of each dot-on pixel in the pixel group, based on the specifiedpriority order and the specified number of dots with regard to the pixelgroup, and actually create a dot at the determined position of eachdot-on pixel.

The image data of the respective pixels in each pixel group arerepresented uniformly by the representative image data. This arrangementensures extremely high-speed specification of the dot number in eachpixel group. The position of each dot-on pixel in each pixel group isdetermined, based on the specified number of dots and the specifiedpriority order of the pixel group. This accordingly ensures high-speeddetermination of the dot-on pixel positions. Selection of the priorityorder for dot formation with regard to each pixel group desirablyprevents appearance of any specific pattern and thus keeps the highpicture quality of an output image.

One preferable procedure processes each original pixel of the image togenerate multiple pixels having identical image data with image data ofthe original pixel, so as to increase a total number of pixels in theimage, and collects the multiple pixels generated from an identicaloriginal pixel to one pixel group. This arrangement ensures output of ahigh-quality image, because of the following reason. The position ofeach dot-on pixel in each pixel group is determined, based on thespecified number of dots and the specified priority order of pixels inthe pixel group. This pixel position determination technique does nottake into account the potential effects of different image data amongthe respective pixels gathered to one pixel group. Determination of dotformation or no-dot formation in the respective pixels of each pixelgroup is, however, affected by a variation in image data among therespective pixels, as well as by the priority order of the respectivepixels representing the tendency of dot formation. A significantvariation of the image data among the respective pixels in one pixelgroup may hinder adequate determination of the dot-on pixel positions.Collection of multiple pixels generated from an identical original pixelto one pixel group enables all the multiple pixels to have identicalimage data in the pixel group. This arrangement enables adequatedetermination of the dot-on pixel positions in each pixel group based onthe specified number of dots and the specified priority of pixels in thepixel group, thus ensuring output of a high-quality image.

The dot number data representing the number of dots to be created ineach pixel group occupies a significantly smaller data capacity,compared with the dot on-off state data representing the dot on-offstate of individual pixels included in each pixel group. Conversion ofthe image data into the dot number data of respective pixel groups thusreduces the required volume of data transfer and attains quick datatransfer. Even when an object image has a large number of pixels, thisarrangement completes data transfer within a short time period and thusenables high-speed image output.

The positions of dot-on pixels in each pixel group are determined, basedon the specified number of dots and one priority order selected withregard to the pixel group among stored multiple priority orders. Evenwhen identical dot numbers are specified for a sequence of adjacentpixel groups, this technique enables dots to be created at differentpixel positions in the respective pixel groups. This avoids dotformation in a regular pattern and thereby prevents deterioration of thepicture quality of a resulting image.

In the image output system and the corresponding image output method ofthe invention, one preferable embodiment stores multiple mappings forconversion of the representative image data of each pixel group into thenumber of dots to be created in the pixel group. The embodiment selectsone mapping for each pixel group among the stored multiple mappings, andspecifies the number of dots to be created in each pixel group, based onthe representative image data of the pixel group and the selectedmapping.

This arrangement ensures output of a high-quality image, because of thefollowing reason. For example, in an area of multiple consecutive pixelgroups having identical image data, the image data of the respectivepixel groups are converted into different dot numbers. While respectivepixel groups have only integral numbers of dots, the average dot numberof the area may be an integral value or non-integral value. The valuerepresenting the average dot number may be varied continually accordingto the occurrence frequencies of the dot numbers in the respective pixelgroups. Adequate setting of the multiple mappings thus enablesconversion of the image data into the adequate dot number data of thearea, thus ensuring output of a high-quality image.

Another advantage of this arrangement based on the selected mapping iseasy specification of the number of dots to be created in each pixelgroup without identifying the respective pixels included in the pixelgroup.

The multiple mappings applied for conversion of image data into dotnumber data may be multiple threshold value sequences. Each thresholdvalue sequence consists of plural threshold values corresponding to thepredetermined number of plural pixels included in each pixel group. Theprocedure selects one threshold value sequence among the stored multiplethreshold value sequences, and sets the number of smaller thresholdvalues in the selected threshold value sequence that are smaller thanthe image data of each pixel group, to the number of dots to be createdin the pixel group.

Storage of the multiple threshold value sequences as the multiplemappings advantageously requires only a small memory capacity.

The plural threshold values of each threshold value sequence may bestored together with information on an order of magnitude of therespective threshold values in the threshold value sequence. Theprocedure refers to the order of magnitude and compares the image dataof each pixel group with the plural threshold values of the selectedthreshold value sequence, so as to specify the number of dots to becreated in the pixel group.

Storage of the information on the order of magnitude of the respectivethreshold values in each threshold value sequence enables quickspecification of the number of smaller threshold values than the imagedata of each pixel group. For example, the comparison shows that athreshold value of an N-th ordinal number of magnitude in a thresholdvalue sequence is smaller than the image data of one pixel group andthat a threshold value of an (N+1)-th ordinal number of magnitude in thethreshold value sequence is greater than the image data of the pixelgroup. In this case, the procedure can specify the number N of smallerthreshold values in the threshold value sequence that are smaller thanthe image data without further comparison of the image data with theremaining threshold values. This means that N dots are to be created inthe pixel group. When each threshold value sequence includes a number ofthreshold values (for example, 20 threshold values), storage of theinformation on the order of magnitude of the respective threshold valuesin each threshold value sequence enables specification of the dot numberaccording to the following procedure. The procedure first compares theimage data with a threshold value having a middle ordinal number ofmagnitude (for example, a threshold value having a 10^(th) ordinalnumber). When the image data is smaller than this selected thresholdvalue, there is no need of comparison between the image data and largerthreshold values than the threshold value having the 10^(th) ordinalnumber. The procedure then compares the image data with a thresholdvalue having a middle ordinal number of magnitude among the smallerthreshold values than the threshold value having the 10^(th) ordinalnumber, for example, a threshold value having a 5^(th) ordinal numberwhen a threshold value having a 1^(st) ordinal number is the smallestthreshold value.

When the image data is larger than this selected threshold value, thereis no need of comparison between the image data and smaller thresholdvalues than the threshold value having the 5^(th) ordinal number. Theimage data is thus compared with threshold values having 6^(th) to9^(th) ordinal numbers of magnitude. In this manner, the procedureperforms the comparison based on the information on the order ofmagnitude of the respective threshold values in each threshold valuesequence and thus quickly specifies the number of dots to be created ineach pixel group.

The information on the order of magnitude of the respective thresholdvalues in each threshold value sequence may be ordinal numbers ofmagnitude allocated to the respective threshold values. In a simplerstorage format, the plural threshold values of each threshold valuesequence may be arranged in the order of magnitude, for example, in anascending order or in a descending order.

This arrangement desirably ensures easy storage of the information onthe order of magnitude of the respective threshold values withoutconsumption of any additional memory capacity.

In the structure of storage of the information on the order of magnitudeof the respective threshold values in each threshold value sequence, thefollowing procedure enables quick specification of the dot number ineach pixel group. When the image data of one pixel group is greater thana preset first threshold value, comparison with the image data of thepixel group may be performed in a descending order of the pluralthreshold values in the selected threshold value sequence. When theimage data of one pixel group is smaller than a preset second thresholdvalue, comparison with the image data of the pixel group may beperformed in an ascending order of the plural threshold values of theselected threshold value sequence.

The procedure starts comparison from the largest threshold value for thelarge image data, while starting comparison from the smallest thresholdvalue for the small image data. This arrangement ensures quickspecification of the number of dots to be created in each pixel group.

In the structure of storage of the information on the order of magnitudeof the respective threshold values in each threshold value sequence,comparison between the image data of each pixel group and the pluralthreshold values of the selected threshold value sequence may start froma threshold value having a selected ordinal number corresponding to amost recently specified dot number.

In general images, image data varies gradually. In many cases, thenumber of dots to be created in one pixel group is thus notsignificantly different from the number of dots to be created in anadjacent pixel group. When N dots are to be created in one pixel group,the number of dots to be created in a subsequently processed pixel groupis generally close to N. The start of comparison from a threshold valuehaving an N-th ordinal number of magnitude or an adjoining ordinalnumber desirably enables quick specification of the dot number.

Another preferable embodiment stores a simplified dither matrix thatincludes the multiple threshold value sequences arranged in a presettwo-dimensional array, as the multiple mappings, and selects onethreshold value sequence corresponding to a position of each pixel groupin the image, among the multiple threshold value sequences stored in thesimplified dither matrix.

Adequate arrangement of the multiple threshold value sequences enablesadequate distribution of the dot numbers to be created in the respectivepixel groups and thus ensures output of a high-quality image.

Like the multiple mappings, the multiple priority orders representingthe tendency of dot formation in each pixel group may also be stored inthe form of a matrix. This embodiment stores a priority order matrixincluding the multiple priority orders of pixels for dot formation ineach pixel group in a preset two-dimensional array. It is desirable thatthe simplified dither matrix and the priority order matrix have anidentical number of rows and an identical number of columns expressed bythe number of pixels.

An image size is generally larger than the size of a matrix. One matrixis thus gradually shifted in position on the image and is repeatedlyapplied to image processing. Different sizes of the simplified dithermatrix and the priority order matrix vary the positional relationbetween the simplified dither matrix and the priority order matrix byeach shift on the image. This may lead to unstable image processing andworsen the picture quality. The identical sizes of the simplified dithermatrix and the priority order matrix, on the other hand, keep the fixedpositional relation by any shift on the image, thus ensuring stableimage processing and desirable picture quality.

In one preferable embodiment, the dot number specification processstores the simplified dither matrix. The simplified dither matrix isgenerated by dividing a dither matrix, which maps threshold values torespective pixels arranged in a two-dimensional array, into multiplegroups corresponding to multiple pixel groups, and includes the multiplethreshold value sequences arranged corresponding to the multiple groups.The pixel position determination process stores the multiple priorityorders representing the tendency of dot formation in the form of apriority order matrix. The priority order matrix is generated bydividing the dither matrix into the multiple groups corresponding to themultiple pixel groups and includes the multiple priority orders arrangedcorresponding to the multiple groups. Here the priority order isspecified with regard to each pixel group based on a magnitude order ofrespective threshold values included in a corresponding group. The pixelposition determination process selects one priority order correspondingto a position of each pixel group in the image, among the multiplepriority orders stored in the priority order matrix, and determines theposition of each dot-on pixel according to the selected priority order.

Application of the identical dither matrix by both the dot numberspecification module and the pixel position determination module enablesadequate determination of the position of each dot-on pixel, thusensuring output of a high-quality image.

In the image output system and the corresponding image output method ofthe invention, both the dot number specification module and the pixelposition determination module may store the identical dither matrix andexecute the respective processes according to the dither matrix.

Another application to attain the object of the invention is an imageprocessing device that causes input image data representing an image togo through a preset series of image processing and thereby generatescontrol data, which is used for control of dot formation by an imageoutput device that creates dots and outputs a resulting processed image.

The image processing device includes: a pixel group setting module thatcollects a predetermined number of plural pixels, among a large numberof pixels constituting the image, to each pixel group; a dot numberspecification module that causes image data of respective pixels in eachpixel group to be represented uniformly by preset representative imagedata and specifies number of dots to be created in each pixel groupaccording to the representative image data; and a number data outputmodule that outputs dot number data representing the specified number ofdots with regard to each pixel group as the control data to the imageoutput device.

There is an image processing method corresponding to the imageprocessing device described above. The present invention is thusdirected to an image processing method that causes input image datarepresenting an image to go through a preset series of image processingand thereby generates control data, which is used for control of dotformation by an image output device that creates dots and outputs aresulting processed image.

The image processing method includes the steps of: (A) collecting apredetermined number of plural pixels, among a large number of pixelsconstituting the image, to each pixel group; (B) causing image data ofrespective pixels in each pixel group to be represented uniformly bypreset representative image data and specifying number of dots to becreated in each pixel group according to the representative image data;and (C) outputting dot number data representing the specified number ofdots with regard to each pixel group as the control data to the imageoutput device.

The image processing device and the corresponding image processingmethod of the invention collect a predetermined number of plural pixelsto each pixel group, specify the number of dots to be created in eachpixel group, and output dot number data representing the specifiednumber of dots as the control data.

The dot number data representing the number of dots to be created ineach pixel group occupies a significantly smaller data capacity,compared with the dot on-off state data representing the dot on-offstate of individual pixels included in each pixel group. Even when anobject image has a large number of pixels, this technique ensures quickdata transfer to the image output device and resulting high-speed imageoutput.

Collection of multiple pixels generated from an identical original pixelto one pixel group enables all the multiple pixels to have identicalimage data in the pixel group. This arrangement enables adequatedetermination of the dot-on pixel positions in each pixel group, thusensuring output of a high-quality image, as discussed above.

In the same manner as the image output system and the correspondingimage output method discussed above, the image processing device and thecorresponding image processing method of the invention may storemultiple mappings for conversion of image data into the dot number andselect one mapping with regard to each pixel group among the storedmultiple mappings to convert the image data of the pixel group into thenumber of dots to be created in the pixel group.

Adequate setting of the multiple mappings enables adequate conversion ofthe image data into the dot number data, thus ensuring output of ahigh-quality image.

The technique of the invention may be actualized by programs that areexecuted by the computer to attain the image output method and the imageprocessing method described above, as well as by recording media inwhich such programs are recorded.

One application of the invention is a program corresponding to the imageoutput method discussed above. The present invention is thus directed toan image output program that is executed by a computer to make imagedata subjected to a preset series of image processing, create dotsaccording to a result of the preset series of image processing, andthereby output an image.

The image output program causes the computer to attain: a first functionof collecting a predetermined number of plural pixels, among a largenumber of pixels constituting the image, to each pixel group; a secondfunction of causing image data of respective pixels in each pixel groupto be represented uniformly by preset representative image data andspecifying number of dots to be created in each pixel group according tothe representative image data; a third function of specifying a priorityorder of pixels for dot formation in each pixel group; a fourth functionof determining position of each dot-on pixel included in each pixelgroup, based on the specified number of dots and the specified priorityorder; and a fifth function of actually creating a dot at the determinedposition of each dot-on pixel.

In one preferable embodiment of the image output program, the secondfunction has the functions of: storing multiple mappings for conversionof the representative image data of each pixel group into the number ofdots to be created in the pixel group; and selecting one mapping foreach pixel group among the stored multiple mappings. The second functionspecifies the number of dots to be created in each pixel group, based onthe representative image data of the pixel group and the selectedmapping.

Another application of the invention is a program corresponding to theimage processing method discussed above. The present invention is thusdirected to an image processing program that is executed by a computerto make image data of an image subjected to a preset series of imageprocessing and thereby generate control data, which is used for controlof dot formation by an image output device that creates dots and outputsa resulting processed image.

The image processing program causes the computer to attain the functionsof: (A) collecting a predetermined number of plural pixels, among alarge number of pixels constituting the image, to each pixel group; (B)causing image data of respective pixels in each pixel group to berepresented uniformly by preset representative image data and specifyingnumber of dots to be created in each pixel group according to therepresentative image data; and (C) outputting dot number datarepresenting the specified number of dots with regard to each pixelgroup as the control data to the image output device.

The technique of the invention may also be actualized by recording mediain which such programs are recorded in a computer readable manner.

The computer reads any of such programs, which may be recorded in therecording media, to attain the various functions described above. Thisensures high-speed image output even when an object image has a largenumber of pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a printing system to explain thegeneral outline of the invention;

FIG. 2 illustrates the configuration of a computer as an imageprocessing device of an embodiment;

FIG. 3 schematically illustrates the structure of a printer as an imageoutput device of the embodiment;

FIG. 4 shows an arrangement of nozzles Nz on respective ink ejectionheads;

FIG. 5 is a flowchart showing an image printing routine executed by theimage processing device of a first embodiment;

FIGS. 6( a) and 6(b) show resolution conversion of image data;

FIG. 7 is a flowchart showing the details of a number data generationprocess executed in the first embodiment;

FIG. 8 shows part of a dither matrix;

FIG. 9 shows a process of determining dot on-off state of individualpixels by referring to the dither matrix;

FIGS. 10( a) and 10(b) conceptually show a process of generating dotnumber data with regard to each pixel group;

FIG. 11 is a flowchart showing the details of a pixel positiondetermination process executed in the first embodiment;

FIGS. 12( a) through 12(d) show a process of determining the positionsof dot-on pixels according to the dot number data in the pixel positiondetermination process of the first embodiment;

FIGS. 13( a) through 13(c) show a process of generating dot number datain a number data generation process of a first modified example of thefirst embodiment;

FIGS. 14( a) through 14(c) show a process of determining the positionsof dot-on pixels according to the dot number data in a pixel positiondetermination process of a second modified example of the firstembodiment;

FIG. 15 is a flowchart showing the details of a number data generationprocess executed in a second embodiment;

FIGS. 16( a) and 16(b) show replacement of tone values of individualpixels included in one pixel group with a mean tone value according tothe presence or the absence of an edge in the pixel group;

FIGS. 17( a) through 17(c) show data formats output from the computer inthe second embodiment;

FIG. 18 shows another data format output from the computer in the secondembodiment;

FIG. 19 is a flowchart showing the details of a pixel positiondetermination process executed in the second embodiment;

FIG. 20 is a flowchart showing an image printing routine executed in athird embodiment;

FIG. 21 conceptually shows a conversion table referred to in aconversion process to large-size, medium-size, and small-size dot datain the third embodiment;

FIGS. 22( a) and 22(b) show a process of generating dot number data fromdot data in a number data generation process of the third embodiment;

FIG. 23 is a flowchart showing the details of the number data generationprocess executed in the third embodiment;

FIG. 24 shows a process of determining the positions of dot-on pixels ofrespective size dots by referring to the dither matrix;

FIG. 25 conceptually shows a conversion table referred to for encodingdot number data of the respective size dots;

FIG. 26 conceptually shows a process of determining the numbers of therespective size dots without specification of dot-on pixel positions inthe number data generation process of the third embodiment;

FIG. 27 is a flowchart showing the details of a pixel positiondetermination process executed in the third embodiment;

FIG. 28 shows a process of determining the positions of dot-on pixels ofthe respective size dots according to the dot number data in the pixelposition determination process of the third embodiment;

FIG. 29 conceptually shows a decoding table referred to in the pixelposition determination process of the third embodiment;

FIG. 30 is a flowchart showing the details of a number data generationprocess executed in an image printing process of a first modifiedexample;

FIG. 31 shows a process of unequivocally determining the numbers ofrespective size dots according to a mean tone value in each pixel group;

FIG. 32 is a flowchart showing the details of a number data generationprocess executed in an image printing process of a second modifiedexample;

FIG. 33 is a flowchart showing the details of a pixel positiondetermination process executed in an image printing process of a thirdmodified example; and

FIG. 34 shows storage of multiple priority orders for individual pixelsincluded in each pixel group.

BEST MODES OF CARRYING OUT THE INVENTION

Some modes of carrying out the invention are discussed below aspreferred embodiments in the following sequence to describe the objects,features, aspects, and advantages of the present invention moreapparently: Some modes of carrying out the invention are discussed belowas preferred embodiments in the following sequence to describe theobjects, features, aspects, and advantages of the present invention moreapparently.

A. General Outline of System B. First Embodiment

B-1. System Configuration

B-2. Schema of Image Printing Process

B-3. Number Data Generation Process in First Embodiment

B-4. Pixel Position Determination Process in First Embodiment

B-5. Modified Examples

C. Second Embodiment

C-1. Number Data Generation Process in Second Embodiment

C-2. Pixel Position Determination Process in Second Embodiment

D. Third Embodiment

D-1. Schema of Image Printing Process in Third Embodiment

D-2. Number Data Generation Process in Third Embodiment

D-3. Pixel Position Determination Process in Third Embodiment

E. Modifications A. General Outline of System

The general outline of a system embodying the invention is describedwith reference to FIG. 1, prior to detailed description of respectiveembodiments. FIG. 1 schematically illustrates the configuration of aprinting system as one mode of an image output control system of theinvention. The printing system includes a computer 10 as an imageprocessing device and a printer 20 as an image output device. Thecomputer 10 loads and executes preset programs and works in combinationwith the printer 20 as the integral printing system. The printer 20creates dots on a printing medium to print an image. The computer 10makes image data, which represent an object image to be printed,subjected to a preset series of image processing to generate and supplycontrol data to the printer 20, which then controls dot formation inrespective pixels according to the received control data.

In a general printing system, the computer converts image data into doton-off state data representing the dot on-off state in respective pixelsconstituting an object image and supplies the dot on-off state data tothe printer. The printer creates dots according to the received doton-off state data to complete a printed image. An increasing number ofpixels included in an object image to be printed naturally leads to anincreasing volume of the dot on-off state data of the respective pixels.The increased data volume undesirably extends the time required for datatransfer from the computer to the printer and thereby the total printingtime. In the printing system 10 of FIG. 1, the computer 10 accordinglyhas a dot number specification module and a number data generationmodule to execute the following series of processing.

The dot number specification module collects a predetermined number ofplural pixels, among a large number of pixels constituting the image, toeach pixel group and specifies the number of dots to be created in eachpixel group according to image data. The dot number data in each pixelgroup is generated by dividing image data into pixel groups anddetermining the dot on-off state in each of the pixel groups. Anotherapplicable procedure may convert image data into data representing thedot on-off state, collect every predetermined number of multiple pixelsto one pixel group, and determine the number of dots to be created ineach pixel group. The predetermined number of multiple pixels collectedto one pixel group may not be mutually adjacent pixels. The number datageneration module sends the dot number data generated with respect toeach of the pixel groups to the printer 20.

The printer 20 shown in FIG. 1 has a priority order storage module, apixel position determination module, and a dot formation module. Thepriority order storage module stores multiple options for the priorityorder of pixels in each pixel group to create dots therein. The pixelposition determination module refers to the priority order storagemodule and determines the positions of dot-on pixels, where dots are tobe created, in each pixel group. A concrete procedure of the pixelposition determination receives dot number data with respect to onepixel group and chooses one from the multiple options for the priorityorder of pixels stored in the priority order storage module. Theprocedure then selects a certain number of pixels specified by the dotnumber data as dot-on pixels according to the chosen priority order,among the predetermined number of multiple pixels included in the pixelgroup. In this manner, the pixel position determination module receivesthe dot number data, chooses the priority order of pixels in each pixelgroup, and determines the positions of dot-on pixels. The dot formationmodule actually creates dots in the dot-on pixels thus determined. Aresulting image is accordingly recorded on a printing medium.

In the printing system of FIG. 1, the computer 10 supplies the dotnumber data representing the number of dots to be created in each pixelgroup, instead of the dot on-off state data of individual pixels, to theprinter 20. This arrangement desirably reduces the total volume of datasupply, compared with the structure of supplying the dot on-off statedata of individual pixels. Even when an image includes a large number ofpixels, this arrangement ensures quick data supply from the computer 10to the printer 20 and thereby enables high-speed printing of a resultingimage. Several embodiments of this printing system are discussed below.

B. First Embodiment

B-1. System Configuration

FIG. 2 illustrates the configuration of a computer 100 as an imageprocessing device in a first embodiment. The computer 100 is a knowncomputer including a CPU 102, a ROM 104, and a RAM 106 interconnectedvia a bus 116.

The computer 100 has a disk controller DDC 109 to read data from, forexample, a flexible disk 124 or a compact disc 126, a peripheralequipment interface PIF 108 to receive and send data from and toperipheral equipment, and a video interface VIF 112 to drive and actuatea CRT 114. The PIF 108 is connected with a color printer 200 describedlater and a hard disk unit 118. Connection of a digital camera 120 or acolor scanner 122 with the PIF 108 enables printing of images taken bythe digital camera 120 or the color scanner 122. Insertion of a networkinterface card NIC 110 to the computer 100 causes the computer 100 toconnect with a communication line 300 and obtain data stored in astorage device 310 linked to the communication line 300.

FIG. 3 schematically illustrates the structure of the color printer 200in this embodiment. The color printer 200 is an inkjet printer that iscapable of creating dots of four color inks, cyan, magenta, yellow, andblack. The inkjet printer may be capable of creating dots of six colorinks, cyan ink of a lower dye density (light cyan ink) and magenta inkof a lower dye density (light magenta ink), in addition to the abovefour color inks. In the description below, cyan ink, magenta ink, yellowink, black ink, light cyan ink, and light magenta ink may be expressedsimply as C ink, M ink, Y ink, K ink, LC ink, and LM ink, respectively.

As illustrated, the color printer 200 has a mechanism of actuating aprint head 241 mounted on a carriage 240 to eject inks and create dots,a mechanism of activating a carriage motor 230 to move the carriage 240back and forth along a shaft of a platen 236, a mechanism of activatinga paper feed motor 235 to feed printing paper P, and a control circuit260 that controls the formation of dots, the shift of the carriage 240,and the feed of the printing paper P.

An ink cartridge 242 for storing the K ink and an ink cartridge 243 forstoring the C, M, and Y inks are attached to the carriage 240. Therespective inks in the ink cartridges 242 and 243 attached to thecarriage 240 are supplied through non-illustrated ink conduits tocorresponding ink ejection heads 244 through 247 of the respectivecolors formed on the bottom face of the print head 241.

FIG. 4 shows an arrangement of inkjet nozzles Nz on the respective inkejection heads 244 through 247. Four nozzle arrays for ejecting the C,M, Y, and K color inks are formed on the bottom face of the respectiveink ejection heads 244 through 247. Each nozzle array includes 48nozzles Nz arranged at a fixed nozzle pitch ‘k’.

The control circuit 260 includes a CPU, a ROM, and a RAM interconnectedvia a bus. The control circuit 260 controls the operations of thecarriage motor 230 and the paper feed motor 235 to regulate main scansand sub-scans of the carriage 240, while controlling ejection of inkdroplets from the respective nozzles at adequate timings according toprint data supplied from the computer 100. The color printer 200 createsink dots of the respective colors at adequate positions on a printingmedium under control of the control circuit 260 and thereby prints aresulting color image.

Any of diverse methods may be applied to eject ink droplets from the inkejection heads of the respective colors. Typical examples of theavailable technique include a method that uses piezoelectric elementsfor ejection of ink droplets and a method that uses heaters located inrespective ink conduits to generate bubbles in the ink conduits forejection of ink droplets. The technique of the invention is alsoapplicable to printers that take advantage of thermal transfer to createink dots on a printing medium and printers that take advantage of staticelectricity to make toners of respective colors adhere to a printingmedium, in addition to the inkjet printers.

In the color printer 200 having the hardware configuration discussedabove, the carriage motor 230 is driven to move the ink ejection heads244 through 247 of the respective colors in a main scanning directionrelative to the printing paper P, while the paper feed motor 235 isactuated to feed the printing paper P in a sub-scanning direction. Thecontrol circuit 260 repeats main scans and sub-scans of the carriage 240and drives nozzles at adequate timings to eject ink droplets accordingto print data. The color printer 200 thus prints a resulting color imageon the printing paper P.

B-2. Schema of Image Printing Process

FIG. 5 is a flowchart showing an image printing routine executed by thecomputer 100 and the printer 200 in the system of the embodiment, so asto make object image data subjected to a preset series of imageprocessing and print a processed image on a printing medium. The firsthalf of the image printing routine is performed by the CPU of thecomputer 100, whereas the latter half is performed by the CPU of thecontrol circuit 260 in the printer 200. The image printing process ofthe embodiment is described below with reference to the flowchart ofFIG. 5.

When the image printing routine starts, the computer 100 first readsobject image data to be converted (step S100). The object image data areRGB color image data in this embodiment, although monochromatic imagedata may be replaced with the color image data.

The input color image data goes through a color conversion process (stepS102). The color conversion process converts the RGB color image dataexpressed by combinations of tone values of the colors R, G, and B intoimage data expressed by combinations of tone values of respective colorsused for printing. As mentioned above, the printer 20 prints an imagewith the four color inks C, M, Y, and K. The color conversion process ofthis embodiment accordingly converts the image data expressed by thetone values of the colors R, G, and B into image data expressed by thetone values of the four colors C, M, Y, and K. The procedure of colorconversion refers to a three-dimensional numerical table called a colorconversion table (LUT). The LUT stores a mapping of the tone values ofthe respective colors C, M, Y, and K to the RGB color image data andthus facilitates and accelerates the color conversion.

The color-converted image data then goes through a resolution conversionprocess (step S104). The resolution conversion process converts theresolution of the image data into a resolution for printing with theprinter 200 (printing resolution). As described previously, sizereduction of pixels to attain printing at the higher resolutioneffectively enhances the picture quality of resulting prints. Theincreased resolution of original image data is, however, not essentialfor the increased printing resolution. The dot printing technique takesonly either of a dot-on state or a dot-off state with respect to eachpixel. Even in the case of variable-size dot printing, only severaltones are expressible in each pixel. The input image data of, forexample, 1 byte, on the other hand, can express 256 tones with respectto each pixel. Namely the number of expressible tones by dot printing ineach pixel is significantly different from the number of expressibletones by the input image data. Setting the higher printing resolutionthan the resolution of the input image data thus leads to improvement inpicture quality of resulting prints. On this ground, the process of stepS104 in the flowchart of FIG. 5 converts the resolution of the inputimage data into the higher printing resolution.

FIGS. 6( a) and 6(b) show an example of the resolution conversionprocess executed in the first embodiment. The prior color conversiongives the image data of the C, M, Y, and K colors. The resolutionconversion process discussed below is applied to the image data in anyof these colors. For the simplicity of explanation, the color is notspecified in the following description.

FIG. 6( a) shows part of image data after color conversion. Thecolor-converted image data has tone values allocated to the respectivepixels arranged in lattice. Each rectangle in FIG. 6( a) represents apixel, and the numeral in the rectangle denotes a tone value allocatedto the pixel. One available method to increase the resolution of theimage data creates new pixels by interpolation of existing pixels. Theresolution conversion process of this embodiment, however, adopts thesimplest technique of dividing each pixel into smaller pixels.

FIG. 6( b) shows division of pixels for conversion of the resolution. Inthe illustrated example, each pixel is divided into four in the mainscanning direction (the horizontal direction in the drawing) and intotwo in the sub-scanning direction (the vertical direction in thedrawing). Namely one pixel is divided into eight smaller pixels. Thebroken lines in each solid rectangle of FIG. 6( b) represent divisionsof each pixel. The tone value given to the original pixel is allocatedto all the smaller divisions of the pixel. Such division of pixelsquadruples the resolution of the image data in the main scanningdirection and doubles in the sub-scanning direction. The multiplicationof resolution may be set arbitrarily according to the requirements.

After conversion of the input resolution into the printing resolution,the computer 100 starts a number data generation process (step S106).The color-converted image data are tone data having tone valuesallocated to the respective pixels. The printer 200 creates dots atadequate densities on pixel positions to print an image. The requiredprocedure accordingly converts the tone data into dot on-off state dataof the respective pixels and transfers the dot on-off state data to theprinter 200. Output of the dot on-off state data in units of individualpixels to the printer 200 undesirably extends the time required for datatransfer with an increase in number of pixels and thereby impedeshigh-speed image printing. The image printing process of this embodimentcollects every predetermined number of multiple pixels to one pixelgroup and transfers dot number data representing the number of dots tobe created in each pixel group to the printer 200 in units of pixelgroups. The dot number data representing the number of dots to becreated in each pixel group may be obtained by converting image datainto dot on-off state data of respective pixels and then collectingevery predetermined number of multiple pixels to one pixel group.Another applicable procedure may first collect every predeterminednumber of multiple pixels to one pixel group and then determine thenumber of dots to be created in each pixel group as discussed later. Thenumber data generation process of step S106 generates the dot numberdata representing the number of dots to be created in each pixel groupand transfers the generated dot number data to the printer 20. Thedetails of the number data generation process will be discussed later.

The CPU of the control circuit 260 in the printer 200 receives the dotnumber data supplied from the computer 100 and starts a pixel positiondetermination process (step S108). As mentioned above, the computer 100supplies the dot number data that represent the number of dots to becreated in each pixel group, instead of the dot on-off state data ofrespective pixels. The pixel position determination process determinesthe positions of dot-on pixels in each pixel group, based on thereceived dot number data with respect to the pixel group. The details ofthe pixel position determination process will be discussed later.

After determination of the positions of dot-on pixels, the printer 200actually creates dots at the dot-on pixel positions thus determined(step S110). As discussed previously with reference to FIG. 3, whilerepeating the main scans and the sub-scans of the carriage 240, theprinter 200 drives and actuates the ink ejection heads to eject the inkdroplets and thereby create ink dots on the printing paper. A resultingimage corresponding to image data is thus printed on the printing paper.

B-3. Number Data Generation Process in First Embodiment

FIG. 7 is a flowchart showing the details of the number data generationprocess executed in the first embodiment. The details of the number datageneration process are described with reference to this flowchart.

The number data generation process first collects a predetermined numberof multiple pixels to a pixel group (step S200). The precedentresolution conversion process has divided one pixel into eight smallerpixels. In this embodiment, the eight smaller pixels obtained bydivision of one pixel are thus collected to one pixel group. Forexample, one pixel on the upper left corner of FIG. 6( a) is dividedinto eight smaller pixels arranged vertically in four columns andhorizontally in two rows on the upper left corner of FIG. 6( b). Theseeight smaller pixels constitute one pixel group. The predeterminednumber of multiple pixels collected to one pixel group may not bemutually adjacent pixels, but may be any pixels having a specifiedpositional relation.

In the case of collecting multiple smaller pixels as divisions of oneidentical pixel to one pixel group, the resolution conversion processmay be omitted from the image printing routine of FIG. 5. In the case ofsuch omission, the terminology ‘pixel group’ in the followingdescription is to be replaced by ‘the pixel prior to resolutionconversion’.

The number data generation process subsequently sets one object pixelfor determination of the dot on-off state (target pixel) among thepredetermined number of multiple pixels collected to one pixel group(step S202). The process then compares the tone value allocated to thetarget pixel with a threshold value stored at the corresponding positionin a dither matrix to determine the dot on-off state with respect to thetarget pixel (step S204). The dither matrix is a two-dimensionalnumerical table that stores multiple threshold values arranged inlattice. The procedure of determining the dot on-off state based on adither matrix is described with reference to FIGS. 8 and 9. FIG. 8 showspart of a dither matrix. This dither matrix stores threshold values,which are selected at random throughout a tone value range of 1 to 255and are allocated to a total of 4096 pixels of 64 pixels in length and64 pixels in width. In this embodiment, the image data are 1-byte dataand the tone value allocated to each pixel is in the range of 0 to 255,so that the threshold values in the dither matrix are selected in thetone value range of 1 to 255. The dither matrix is not restricted to thesize of 64 pixels in both length and width as in the example of FIG. 8,but may have any desired size having different numbers of pixels inlength and in width or having the same numbers of pixels in both lengthand width.

FIG. 9 conceptually shows determination of the dot on-off state withrespect to each target pixel, based on the dither matrix. The procedureof determining the dot on-off state first compares the tone value ofeach target pixel with a threshold value stored at the correspondingposition in the dither matrix. Each arrow of thin broken line in FIG. 9represents comparison between the tone value of each target pixel and athreshold value stored at the corresponding position in the dithermatrix. When the tone value of the target pixel is greater than thecorresponding threshold value in the dither matrix, the processdetermines formation of a dot in the target pixel. When the tone valueof the target pixel is smaller than the corresponding threshold value inthe dither matrix, on the contrary, the process determines formation ofno dot in the target pixel. In the example of FIG. 9, the tone valueallocated to a pixel on the upper left corner of image data is ‘97’,while the threshold value stored at the corresponding position in thedither matrix is ‘1’. Namely the process determines formation of a dotin this pixel. Each arrow of solid line in FIG. 9 represents a processof determining formation of a dot in a target pixel and writing theresult of determination at a corresponding position in a memory. Anadjacent pixel on the right side of the upper left pixel also has thetone value ‘97’, while the corresponding threshold value in the dithermatrix is ‘177’. The threshold value is greater than the tone value, sothat the process determines formation of no dot in this pixel. In thismanner, the process refers to the dither matrix and determines formationor no-formation of a dot in the target pixel at step S204 in theflowchart of FIG. 7.

The number data generation process then determines whether the aboveseries of processing has been completed with respect to all the pixelsin the pixel group (step S206). When the pixel group still has anyunprocessed pixel (step S206: no), the process returns to step S202 andrepeats the subsequent series of processing. When the dot on-off statehas been determined with respect to all the pixels in the pixel group(step S206: yes), the process detects the number of dots to be createdin the pixel group as dot number data and stores the dot number datawith respect to the processed pixel group into the memory (step S208).In the illustrated example of FIG. 9, three pixels are determined asdot-on pixels in the pixel group on the upper left corner of the imagedata. Namely the dot number data representing the dot number ‘3’ isstored into the memory.

After conclusion of the processing with regard to one pixel group, theprocess subsequently determines whether the processing has beencompleted with regard to all the pixels included in image data (stepS210). When there is any unprocessed pixel, the process returns to stepS200 to set a next pixel group, repeats the subsequent series ofprocessing to generate dot number data with regard to the next pixelgroup, and stores the dot number data (step S208). When the processinghas been completed with regard to all the pixels in the image data (stepS210: yes), the process outputs the dot number data stored in units ofpixel groups to the printer 200 (step S212). Here terminates the numberdata generation process shown in FIG. 7.

FIG. 10( a) conceptually shows dot number data obtained by the numberdata generation process discussed above. Each of multiple rectanglesrepresents a pixel group, and the numeral shown in each pixel groupdenotes storage of the number of dots to be created in the pixel group.In the system of this embodiment, the computer 100 convertscolor-converted image data into dot number data as shown in FIG. 10( a)and outputs only the dot number data stored with respect to each pixelgroup to the printer 200. Output of only the dot number data desirablyreduces the data volume and thus ensures higher-speed data output,compared with the procedure of outputting the dot on-off state data ofindividual pixels, as discussed below.

FIG. 10( b) shows the dot on-off state of respective pixels in multiplepixel groups. The thin broken lines in FIG. 10( b) show that each pixelgroup consists of multiple pixels. Each square filled with slant linesrepresents a dot-on pixel where a dot is to be created.

It is assumed that the computer 100 outputs the dot on-off state data ofindividual pixels as shown in FIG. 10( b) to the printer 200. When thereis only one type of dot, each pixel takes only either of the two states,that is, the dot-on state or the dot-off state. The data volume requiredfor each pixel is accordingly 1 bit. Since each pixel group consists ofeight pixels, the data volume output to the printer 200 is 8 bits withrespect to each pixel group.

The procedure of this embodiment, on the other hand, outputs the dotnumber data representing the number of dots to be created in each pixelgroup. The number of dots to be created in one pixel group varies in therange of 0 to 8. The dot number data with respect to each pixel groupthus requires only 4 bits. This desirably halves the data volume,compared with output of the dot on-off state data of individual pixels.Output of the dot number data in units of pixel groups thus attainshigh-speed data transfer to the printer 200. The dot number datatransferred from the computer 100 go through the pixel positiondetermination process executed by the printer 200 to be converted intothe dot on-off state data of individual pixels, as discussed below.

B-4. Pixel Position Determination Process in First Embodiment

FIG. 11 is a flowchart showing the details of the pixel positiondetermination process executed in the image printing routine of thefirst embodiment. The CPU of the control circuit 260 in the printer 200receives the dot number data transferred in units of pixel groups fromthe computer 100 and executes the pixel position determination process.FIG. 12 conceptually shows conversion of the dot number datarepresenting the number of dots to be created in each pixel group intothe dot on-off state data of individual pixels by the pixel positiondetermination process. The details of the pixel position determinationprocess are described below with reference to FIGS. 11 and 12.

The pixel position determination process first selects a target pixelgroup as an object of processing (step S300 in the flowchart of FIG. 11)and acquires the dot number data representing the number of dots to becreated in the selected target pixel group (step S302). FIG. 12( a)conceptually shows the dot number data transferred in units of pixelgroups from the computer 100. In this illustrated example, a pixel groupon the upper left corner is selected as a target pixel group goingthrough the pixel position determination process. The process acquiresthe dot number data ‘3’ as the number of dots to be created in theselected target pixel group at step S302 in FIG. 11.

The process subsequently refers to a priority order of pixels in thetarget pixel group for dot formation and determines dot-on pixels (stepS304). In this example, the dither matrix is used to set the priorityorder of dot formation. As discussed previously with reference to FIG.8, threshold values are set at respective pixel positions in the dithermatrix. Determination of the dot on-off state in one pixel compares thetone value of image data allocated to the pixel with a threshold valueat the corresponding position in the dither matrix. When the tone valueis greater than the threshold value, the pixel is determined as a dot-onpixel. The pixel having the smaller setting of the threshold value inthe dither matrix has the greater tendency of dot formation. The dithermatrix is thus assumed to show the priority order of respective pixelsfor dot formation. The procedure of this embodiment takes advantage ofthe characteristics of the dither matrix and uses the dither matrix toset the priority order of pixels in the target pixel group for dotformation.

In the illustrated example of FIG. 12( a), the pixel group on the upperleft corner is selected as the target pixel group to be processed. Theprocedure reads threshold values in the dither matrix stored atcorresponding positions of the respective pixels included in theselected target pixel group. FIG. 12( b) shows threshold values readfrom the corresponding pixel positions in the dither matrix shown inFIG. 8. Dots are created in the order of pixels having the smallerthreshold values. The number of dots to be created in the currentlyprocessed target pixel group is 3 as shown in FIG. 12( a). The positionsof dot-on pixels are determined according to the priority order ofpixels for dot formation as shown in FIG. 12( c). In this illustratedexample of FIG. 12( c), the pixel of the smallest threshold valuesurrounded by the solid line, the pixel of the second smallest thresholdvalue surrounded by the broken line, and the pixel of the third smallestthreshold value surrounded by the one-dot chain line are determined asdot-on pixels.

After determination of the positions of dot-on pixels in the targetpixel group selected as the object of processing, it is determinedwhether the processing has been completed with regard to all the pixelgroups (step S306 in the flowchart of FIG. 11). When there is anyunprocessed pixel group (step S306: no), the pixel positiondetermination process returns to step S300 to set a next pixel group andrepeats the subsequent series of processing with respect to the nextpixel group. The dot number data representing the number of dots to becreated in each pixel group as shown in FIG. 12( a) are accordinglyconverted to the dot on-off state data of individual pixels as shown inFIG. 12( d). The squares filled with slant lines in FIG. 12( d)represent the dot-on pixels. On conclusion of the processing with regardto all the pixel groups (step S306: yes), the program exits from thepixel position determination process shown in FIG. 11 and returns to theimage printing routine of FIG. 5.

The above description regards the image printing process executed in thefirst embodiment, as well as the details of the number data generationprocess and the pixel position determination process included in theimage printing process. In the image printing process of the firstembodiment, the computer 100 transfers the dot number data representingthe number of dots to be created in each pixel group to the printer 200,instead of the dot on-off state data of the individual pixels. Thisarrangement significantly reduces the total volume of data transfer.Even when an image includes a large number of pixels, the procedure ofthe first embodiment ensures quick data transfer and high-speed imageprinting.

As described above, the dither matrix referred to in the number datageneration process executed by the computer 100 is identical with thedither matrix referred to in the pixel position determination processexecuted by the printer 200. Such setting ensures complete restorationof the positions of dot-on pixels even in the case of transfer of onlythe dot number data from the computer 100 to the printer 200. As clearlyunderstood from the comparison between FIG. 10( b) and FIG. 12( d), thepositions of dot-on pixels based on the determination of the dot on-offstate of the individual pixels executed by the computer 100 areperfectly matched with the positions of dot-on pixels determined by theprinter 200. This proves complete restoration of the positions of dot-onpixels. The arrangement of the first embodiment thus enables the printer200 to accurately determine the positions of dot-on pixels, whileensuring quick transfer of the dot number data from the computer 100 tothe printer 200. This leads to high-speed printing of a high-qualityimage.

B-5. Modified Examples

The procedure of the first embodiment may be modified in various ways.Some examples of possible modification are discussed briefly below.

(1) First Modified Example

As discussed above with reference to FIGS. 7 through 10, the number datageneration process of the first embodiment converts image data into doton-off state data of individual pixels, further converts the dot on-offstate data into dot number data representing the number of dots to becreated in each pixel group, and transfers the dot number data to theprinter 200. Namely the computer 100 determines the dot on-off statewith specification of dot-on pixel positions. The information onspecification of dot-on pixel positions is, however, omitted from thetransferred data, and only the information on the number of dots to becreated in each pixel group is transferred to the printer 200. In viewof such omission, the number data generation process executed in a firstmodified example thus generates only the dot number data representingthe number of dots to be created in each pixel group withoutspecification of dot-on pixel positions.

FIG. 13 shows the outline of the number data generation process executedin the first modified example. FIG. 13( a) shows a simplified dithermatrix referred to in the number data generation process of the firstmodified example. The standard dither matrix referred to in the numberdata generation process of the first embodiment has the settings ofthreshold values corresponding to respective pixel positions (see FIG.8). In the simplified dither matrix referred to in the first modifiedexample, on the other hand, threshold values are not one-to-one mappedto the respective pixel positions but are collectively mapped to therespective pixel groups. Namely a set of multiple threshold values isone-to-one mapped to one pixel group. The number of multiple thresholdvalues mapped to each pixel group is identical with the number ofmultiple pixels included in each pixel group. In the illustrated exampleof FIG. 13( a), a set of eight threshold values{255,212,177,170,109,58,42,1} is mapped to a pixel group on the upperleft corner of the simplified dither matrix. Similarly another set ofeight threshold values {242,223,186,161,79,70,48,5} is mapped to anadjacent pixel group on the right side.

The number data generation process of the first modified examplecompares the image data in each pixel group with a corresponding set ofthreshold values and thereby determines the number of dots to be createdin each pixel without specification of dot-on pixel positions. Forconvenience of explanation, it is here assumed that target image data tobe processed is identical with the example of image data processed inthe first embodiment (see FIG. 6( b)). In the pixel group on the upperleft corner of the image data, all the pixels have an identical tonevalue ‘97’. The pixel group at the corresponding position in thesimplified dither matrix stores the set of eight threshold values{255,212,177,170,109,58,42,1}. Among these eight threshold values, onlythree threshold values {58,42,1} are smaller than the tone value ‘97’ ofthe pixel group. Namely three dots are to be created in this pixelgroup. Part of the threshold values are surrounded by the broken line inFIG. 13( b). This shows that these threshold values are smaller than thetone value of the pixel group. The set of multiple threshold values arestored corresponding to each pixel group and are compared with the tonevalue of the pixel group. This procedure readily determines the numberof dots to be created in the pixel group without specifying the dot-onpixel positions in the pixel group. This series of processing isrepeated for all the pixel groups and determines the numbers of dots tobe created in the respective pixel groups as shown in FIG. 13( c).

The simplified dither matrix referred to in the first modified exampleis equivalent to the standard dither matrix referred to in the firstembodiment. Each set of multiple threshold values stored correspondingto each pixel group in the simplified dither matrix of FIG. 13 isobtained by gathering threshold values stored at respective pixelpositions in the dither matrix of FIG. 8 to one pixel group. Suchequivalency of the simplified dither matrix to the standard dithermatrix gives the same result of the dot number specification based onthe simplified dither matrix without specification of dot-on pixelpositions as the result of the dot number specification based on thestandard dither matrix with specification of dot-on pixel positions.This is proved by the fact that the dot number data obtained byprocessing the image data with the standard dither matrix (see FIG. 10(a)) are perfectly matched with the dot number data obtained byprocessing the same image data with the simplified dither matrix (seeFIG. 13( c)).

The procedure of the first modified example determines the number ofdots to be created in each pixel group by simple comparison between theset of multiple threshold values stored corresponding to each pixelgroup and the tone value of the image data in the pixel group. Thisarrangement does not require comparison between the threshold values andthe tone values of the image data at individual pixel positions in eachpixel group, thus more quickly generating the dot number datarepresenting the number of dots to be created in each pixel group.

The procedure of the first modified example specifies only the number ofthe threshold values that are smaller than the tone value of the imagedata in each pixel group. Arrangement of the threshold values in theorder of magnitude in each pixel group further enhances the processingspeed. In the illustrated example of FIG. 13, in the pixel group on theupper left corner, the tone value of the image data is equal to ‘97’,while the set of the threshold values are arranged in the order ofmagnitude {255,212,177,170,109,58,42,1}. These threshold values may bearranged either in a descending order or in an ascending order. The tonevalue of the image data varies in the range of 0 to 255, so that thetone value ‘97’ is relatively small. This relatively small tone value isthus compared with the threshold values in the ascending order. The tonevalue ‘97’ is first compared with the smallest threshold value ‘1’ andis determined to be greater than the threshold value ‘1’. The tone value‘97’ is then compared with the adjacent second-smallest threshold value‘42’ and is also determined to be greater than the threshold value ‘42’.The tone value ‘97’ is subsequently compared with the adjacentthird-smallest threshold value ‘58’. In this manner, the tone value issuccessively compared with the threshold values in the ascending order.The tone value ‘97’ is compared with the threshold value ‘109’ and isdetermined to be smaller than the threshold value ‘109’. Since thethreshold values in one pixel group are arranged in the order ofmagnitude, it is obvious that the subsequent threshold values aregreater than the tone value once the threshold value exceeds the tonevalue. No further comparison is accordingly required. Although eachpixel group stores eight threshold values, the dot number data isobtained by comparison of the tone value with only the four thresholdvalues in this pixel group.

When the tone value of the image data is relatively large, on the otherhand, the tone value is compared with the threshold values in thedescending order. For example, when the tone value of the image data is‘200’ in the above example, the comparison with the tone value startsfrom the largest threshold value ‘255’ and then goes to the adjacentsecond-largest threshold value ‘212’, and the adjacent third-largestthreshold value ‘177’. The dot number data in the pixel group isobtained by comparison of the tone value with only these three thresholdvalues. As described above, storage of the threshold values in the orderof magnitude in each pixel group ensures quick determination of thenumber of dots to be created in the pixel group.

(2) Second Modified Example

The pixel position determination process of the first embodimentdiscussed above receives the dot number data representing the number ofdots to be created in each pixel group, refers to the dither matrix, anddetermines the positions of dot-on pixels in each pixel group (see FIG.12). The threshold values of the individual pixels are, however, notessential for determination of the positions of dot-on pixels accordingto the dot number data. The only requirement is a priority order of therespective pixels in each pixel group for dot formation. In view of suchrequirement, the pixel position determination process in a secondmodified example refers to a matrix storing a priority order of pixels(hereafter referred to as the priority order matrix), instead of thedither matrix, to determine the positions of dot-on pixels.

FIG. 14 conceptually shows determination of the positions of dot-onpixels according to a priority order matrix in the pixel positiondetermination process of the second modified example. FIG. 14( a) showsa priority order matrix. Each rectangle of the thick solid linerepresents a pixel group. Each pixel group is divided into eight pixelsas shown by the thin broken lines. The numerals shown in the respectivepixels denote a priority order of the pixels in each pixel group for dotformation (that is, an order of dot formation).

Application of the priority order matrix facilitates determination ofthe positions of dot-on pixels according to the dot number data. The dotnumber data used for the description of this example are those shown inFIG. 12( a) and are identical with the dot number data used for thedescription of the pixel position determination process in the firstembodiment. According to the dot number data of FIG. 12( a), the numberof dots to be created in a pixel group on the upper left corner is 3.The procedure of the second modified example accordingly selects threepixels having first to third priority numbers in the upper left pixelgroup of the priority order matrix shown in FIG. 14( a) and determinesthe positions of dot-on pixels. FIG. 14( b) shows such selection ofthree pixels to determine the positions of dot-on pixels. The solidlines surrounding the numerals in the pixels mean that the pixels areselected. This series of processing is repeated with regard to all thepixel groups to determine all the positions of dot-on pixels as shown inFIG. 14( c). The squares filled with slant lines in FIG. 14( c)represent dot-on pixels.

The procedure of this modified example compares the priority number ofeach pixel included in a selected target pixel group with the dot numberdata representing the number of dots to be created in the pixel group.Selection of pixels having the priority numbers of not greater than thedot number data readily determines the positions of dot-on pixels in thepixel group. This method does not need to count up the number of pixelsselected for dot formation according to the dot number data, thusfacilitating determination of the positions of dot-on pixels.

The largest value stored in the priority order matrix (that is, thelargest priority number allocated to the pixel) is the number of pixelsincluded in one pixel group and is thus significantly smaller than thethreshold values stored in the dither matrix. Namely the priority ordermatrix occupies a remarkably smaller storage capacity than the dithermatrix. The printer executing the pixel position determination processmay not have a sufficient storage capacity. The use of the priorityorder matrix to determine the positions of dot-on pixels advantageouslysaves the storage capacity of the printer.

The priority order matrix shown in FIG. 14( a) corresponds to the dithermatrix referred to in the number data generation process fordetermination of the dot on-off state in the respective pixels. Asmentioned previously with regard to the pixel position determinationprocess of the first embodiment, the threshold values set in the dithermatrix represent the priority order of dot formation. The priority orderset in the priority order matrix shown in FIG. 14( a) is identical withthe ascending order of the threshold values in the dither matrixallocated to the respective pixels in the pixel group. Setting thepriority order matrix corresponding to the dither matrix referred to inthe number data generation process ensures adequate determination of thepositions of dot-on pixels according to the priority order matrix. Asclearly understood from the comparison between FIG. 14( c) and FIG. 10(b), the positions of dot-on pixels determined according to the priorityorder matrix are completely matched with the positions of dot-on pixelsaccording to the determination of the dot on-off state of the individualpixels. This proves adequate determination of the positions of dot-onpixels according to the priority order matrix.

As mentioned above, the number data generation process is required tospecify only the number of dots to be created in each pixel group anddoes not demand information on the positions of dot-on pixels, that is,information regarding which pixels in each pixel group are dot-onpixels. The number data generation process may thus refer to thesimplified dither matrix to generate the dot number data, instead of thestandard dither matrix. The number data generation process based on thestandard dither matrix specifies not only the number of dots to becreated in each pixel group but the positions of dot-on pixels. Omissionof the information on the positions of dot-on pixels from the standarddither matrix gives the simplified dither matrix. The modified numberdata generation process based on the simplified dither matrix thusspecifies only the number of dots to be created in each pixel group.

The pixel position determination process receives the dot number datarepresenting the number of dots to be created in each pixel group and isrequired to determine only the positions of dot-on pixels. The pixelposition determination process may thus refer to the priority ordermatrix to determine the positions of dot-on pixels, instead of thestandard dither matrix. Omission of the information used forspecification of the number of dots to be created in each pixel groupfrom the dither matrix gives the priority order matrix.

Both the simplified dither matrix and the priority order matrix haveless information volumes than the standard dither matrix. Thecombination of the simplified dither matrix and the priority ordermatrix gives the equivalent volume of information to that of thestandard dither matrix. Namely a simplified dither matrix and a priorityorder matrix can be generated corresponding to an original standarddither matrix of any arrangement. The generation of the dot number dataaccording to the simplified dither matrix and the subsequentdetermination of the positions of dot-on pixels according to thepriority order matrix ensure formation of dots in the identicalarrangement with the result of dot formation according to the originalstandard dither matrix.

C. Second Embodiment

The procedure of the first embodiment discussed above divides one pixelinto multiple smaller pixels and gathers the multiple smaller pixels asdivisions of an identical original pixel to a pixel group. Division ofone pixel into multiple smaller pixels is required, for example, when animage is printed at a higher resolution than the resolution of inputimage data. In the procedure of the first embodiment, the respectivepixels included in one pixel group have an identical tone value. Thetechnique of the invention is, however, also applicable to a pixel groupof multiple pixels having different tone values. For example, when theprinting resolution is set identical with the resolution of the inputimage data and one pixel group includes multiple pixels of image data,the multiple pixels included in the pixel group may have different tonevalues. In another example, when additional pixels are newly created forprinting an image at a higher resolution than the resolution of inputimage data and the tone values of the newly created pixels are specifiedby interpolation, multiple pixels included in one pixel group havedifferent tone values. In still another example, when the printingresolution is only slightly higher than the resolution of input imagedata and one pixel is divided into multiple smaller pixels, one pixelgroup may include smaller pixels as divisions of different originalpixels. In such cases, the multiple pixels included in one pixel groupmay have different tone values. Application of the technique of theinvention to such cases is described below as a second embodiment.

C-1. Number Data Generation Process in Second Embodiment

FIG. 15 is a flowchart showing a number data generation process executedin the second embodiment. Like the number data generation process of thefirst embodiment, the number data generation process of the secondembodiment is executed by the CPU of the computer 100 in the course ofthe image printing process shown in FIG. 5.

The number data generation process of the second embodiment firstcollects a predetermined number of multiple pixels to one pixel group(step S400). As in the description of the first embodiment, each pixelgroup includes eight pixels, that is, two rows in the horizontaldirection and four columns in the vertical direction. This is onlyillustrative and not restrictive at all, and each pixel group mayinclude any number of rows and any number of columns.

The number data generation process then determines whether the pixelgroup includes an edge (step S402). The procedure of this embodimentdetermines that the pixel group includes an edge when a tone differencebetween a largest tone value and a smallest tone value among the tonevalues allocated to the multiple pixels included in the pixel group isnot less than a preset level. This method is, however, not restrictiveat all, and any other suitable method is applicable to detection of anedge.

When it is determined that no edge is included in the pixel group (stepS402: no), the process calculates a mean tone value of the pixel groupand replaces the tone values of the respective pixels in the pixel groupwith the calculated mean tone value (step S404). The respective pixelsincluded in one pixel group accordingly have an identical tone value.The number of dots to be created in the pixel group is thus determinedaccording to the same procedure as the first embodiment described above.

FIG. 16 shows replacement of the tone values of the respective pixels ineach pixel group with the mean tone value in the case of detection of noedge in the pixel group. FIG. 16( a) shows tone values allocated torespective pixels in pixel groups. A pixel group on the upper leftcorner has a maximum tone value ‘100’ and a minimum tone value ‘97’.There is a relatively small tone difference of ‘3’. A lower right pixelgroup, on the other hand, has a maximum tone value ‘132’ and a minimumtone value ‘99’. There is a significantly large tone difference of ‘33’.One concrete procedure presets an adequate threshold value (for example,a tone value ‘20’) and detects the presence of an edge in one pixelgroup when the tone difference in the pixel group is greater than thepreset threshold value. The procedure detects no edge in the pixel groupwhen the tone difference in the pixel group is smaller than the presetthreshold value. In response to detection of no edge, the procedurecalculates the mean tone value of the pixel group and replaces the tonevalues of all the pixels included in the pixel group with the calculatedmean tone value.

FIG. 16( b) shows replacement of the tone values of the respectivepixels included in one pixel group with the calculated mean tone value,in response to detection of no edge in the pixel group. In theillustrated example, the upper left pixel group, an adjacent pixel groupon the right, and another adjacent pixel group immediately below haverelatively small tone differences. All the pixels in the respectivepixel groups are thus replaced by the individually calculated mean tonevalues ‘99’, ‘103’, and ‘94’. With regard to each pixel group having thetone values of the respective pixels replaced with the mean tone value,all the pixels in the pixel group have the identical tone value. Theprocess thus determines the number of dots to be created in such a pixelgroup according to the procedure of the first embodiment discussed above(step S406 in the flowchart of FIG. 15). The lower right pixel group, onthe other hand, has a relatively large tone difference of ‘33’, whichdetects the presence of an edge. The process does not replace the tonevalues with the mean tone value in this pixel group but compares thetone values of the individual pixels in the pixel group with thecorresponding threshold values in the dither matrix to determine the doton-off state of the respective pixels (see FIG. 9) and stores theresults of determination regarding the dot on-off state of therespective pixels (step S408 in FIG. 15).

After conclusion of the processing with regard to one pixel group, theprocess subsequently determines whether the processing has beencompleted with regard to all the pixels included in image data (stepS410). When there is any unprocessed pixel (step S410: no), the processreturns to step S400 to set a next target pixel group of multiple pixelsand repeats the subsequent series of processing. When all the pixelshave been collected to pixel groups and processed (step S410: yes), theprocess outputs the dot number data, which have been stored at stepS406, with regard to the pixel groups with no edge to the printer 200,while outputting the dot on-off state data of individual pixels, whichhave been stored at step S408, with regard to the pixel groups with anedge to the printer 200 (step S412). Namely the dot number data of therespective pixel groups and the dot on-off state data of the individualpixels in the pixel groups are simultaneously output to the printer 200.The number data generation process of the second embodiment outputsthese data in the following formats to discriminate the dot number datafrom the dot on-off state data of the individual pixels.

FIG. 17 shows the formats of the data output from the computer 100 tothe printer 200 in the second embodiment. The dot number datarepresenting the number of dots to be created in each pixel group areoutput as 4-bit data with regard to each pixel group as shown in FIG.17( a). Each pixel group includes eight pixels, so that the dot numbervaries only in the range of 0 to 8. The data capacity of 4 bits is thussufficient to express the number of dots. The dot on-off state datarepresenting the dot on-off state of the individual pixels are output inthe format shown in FIG. 17( b). The first four bits express a valueselected in a range of 9 to 15 (the value ‘9’ in the illustrated exampleof FIG. 17( b)), and the subsequent eight bits express the settings ofthe dot on-off state of the individual pixels. Since the dot numbervaries only in the range of 0 to 8, setting the value of or over 9 inthe 4-bit data head means that the subsequent 8-bit data does not showthe dot number but represents the dot on-off state of the individualpixels. The respective bits of the 8-bit data may be allocated to theindividual pixels, for example, in an order of FIG. 17( c). The doton-off state data representing the dot on-off state of respective pixelsin each pixel group are output in this format. In the case ofapplication of the formats of FIG. 17 to the data output, 4-bit dotnumber data are transferred to the printer 200 with regard to the pixelgroups with no edge, while 12-bit dot on-off state data are transferredto the printer 200 with regard to the pixel groups with an edge.

This method is, however, not restrictive at all, and any other suitablemethod may be applied to output the dot number data and the dot on-offstate data of the individual pixels simultaneously. One applicablemethod may additionally use an identification bit. For example, theidentification bit set equal to ‘0’ means that subsequent 4-bit data isdot number data as shown in FIG. 18( a), whereas the identification bitset equal to ‘1’ means that subsequent 8-bit data is dot on-off statedata of individual pixels as shown in FIG. 18( b). In the case ofapplication of the formats of FIG. 18 to the data output, 5-bit dotnumber data are transferred to the printer 200 with regard to the pixelgroups with no edge, while 9-bit dot on-off state data are transferredto the printer 200 with regard to the pixel groups with an edge.

As clearly understood from the comparison between the data transfermethod of FIG. 17 and the data transfer method of FIG. 18, with regardto pixel groups with no edge, the volume of data transfer according tothe method of FIG. 17 is less than that according to the method of FIG.18. With regard to pixel groups with an edge, on the other hand, thevolume of data transfer according to the method of FIG. 18 is less thanthat according to the method of FIG. 17. In the case of a high ratio ofpixel groups with an edge, the method of FIG. 18 with the identificationbit is suitable for the data transfer. In the case of a low ratio ofpixel groups with an edge, on the other hand, the method of FIG. 17 issuitable for the data transfer. The pixel groups with an edge generallyoccupy a relatively low ratio, so that the method of FIG. 17 ispreferable to attain the quicker data transfer.

As described above, the dot number data of the respective pixel groupsand the dot on-off state data of the individual pixels aresimultaneously output to the printer 200 at step S412 in the flowchartof FIG. 15. On completion of data output with regard to all the pixelgroups, the program exits from the number data generation process of thesecond embodiment shown in FIG. 15 and returns to the image printingroutine.

C-2. Pixel Position Determination Process in Second Embodiment

As described above, in the procedure of the second embodiment, the dotnumber data and the dot on-off state data of the individual pixels aresent simultaneously from the computer 100. The printer 200 determinesthe positions of dot-on pixels according to the following method. In thedescription below, it is assumed that the formats of FIG. 17 are adoptedfor the data transfer.

FIG. 19 is a flowchart showing a pixel position determination processexecuted in the second embodiment to determine the positions of dot-onpixels. The pixel position determination process first inputs data ofthe 4-bit volume (step S500) and determines whether the input 4-bit datarepresents a value of or over 9 (step S502). As mentioned above, eachpixel group includes eight pixels, and the dot number varies only in therange of 0 to 8. When the input 4-bit data represents a value of or over9, it is determined that the input data is not the dot number data butthe subsequent data is dot on-off state data of individual pixels. Whenthe input 4-bit data represents a value of or below 8, on the otherhand, it is determined that the input data is the dot number datarepresenting the number of dots to be created in one pixel group.

When the input 4-bit data does not represent a value of or over 9 (stepS502: no), the process regards the input 4-bit data as dot number dataand determines the positions of dot-on pixels in the pixel groupaccording to the same procedure as that of the first embodiment (stepS504). When the input 4-bit data represents a value of or over 9 (stepS502: yes), on the other hand, the process reads the subsequent 8-bitdata and regards the subsequent 8-bit data as the dot on-off state dataof the individual pixels to determine the positions of dot-on pixels(step S506).

After determination of the positions of dot-on pixels in one pixelgroup, it is determined whether the processing has been completed withregard to all the pixel groups (step S508). When there is anyunprocessed pixel group, the pixel position determination processreturns to step S500 and repeats the subsequent series of processingwith respect to a next pixel group. The above series of processing isrepeated until the positions of dot-on pixels are determined with regardto all the pixel groups. On conclusion of the processing with regard toall the pixel groups, the program exits from the pixel positiondetermination process of the second embodiment shown in FIG. 19 andreturns to the image printing routine.

The image printing process of the second embodiment executes the numberdata generation process shown in FIGS. 15 through 18 and the pixelposition determination process shown in FIG. 19. Even when multiplepixels included in one pixel group have different tone values, the imageprinting process of the second embodiment transfers the dot number datawith regard to the pixel groups with no edge. This arrangement ensuresquick data transfer to the printer 200 and thereby high-speed imageprinting.

The image printing process of the second embodiment transfers the doton-off state data of the individual pixels with regard to the pixelgroups with an edge. An increase in ratio of the pixel groups with anedge extends the time required for data transfer to the printer 200. Onepossible measure elevates the criterion for detection of edges andthereby reduces the potential for edge detection. This shortens the timerequired for data transfer and thereby enables high-speed imageprinting. With regard to the pixel groups with no edge, on the otherhand, the tone values of the individual pixels in the pixel group arereplaced with the calculated mean tone value. The excessive elevation ofthe criterion for edge detection (the excessively reduced potential foredge detection) to shorten the time of data transfer may lead todeterioration of the printing quality.

When the high picture quality is demanded for a resulting printed image,the printing resolution is typically set higher than the resolution ofthe input image data. Enhancement of the resolution is attained bydivision of original pixels into smaller pixels according to theresolution conversion process (step S104 in the flowchart of FIG. 5) orby interpolation for creation of additional pixels. In either case, thetone value gently varies in most pixel groups. In the image with thedemand for the high printing quality, no edge is thus detected in mostpixel groups even without elevation of the criterion for edge detection.This ensures quick data transfer with keeping the demanded high printingquality.

When the demanded printing resolution is not so high but issubstantially equivalent to the resolution of the input image data, onthe other hand, there is a high potential for edge detection inrespective pixel groups. Elevation of the criterion for the reducedpotential for edge detection is thus required to shorten the time ofdata transfer to the printer 200. In the case of setting the printingresolution to a relatively low level, the user generally givespreference to the high-speed printing over the printing quality. Therelatively poor picture quality due to the elevated criterion for edgedetection accordingly does not cause any significant problem.

D. Third Embodiment

In the first and the second embodiments discussed above, each pixeltakes only either of the two tone values corresponding to the dot-onstate and the dot-off state. Some printers may, however, be capable ofvarying the size of dots or varying the density of ink used for dotformation to express a greater number of tone values in individualpixels. The technique of the invention is effectively applicable to suchmulti-valued printers. Application of the technique of the invention toa multi-valued printer is described below as a third embodiment.

D-1. Schema of Image Printing Process in Third Embodiment

FIG. 20 is a flowchart showing an image printing routine executed in thethird embodiment. The primary difference of the image printing processof the third embodiment from the image printing process of the firstembodiment discussed above with reference to FIG. 5 is conversion ofcolor-converted data into large-size, medium-size, and small-size dotdata. The image printing process of the third embodiment is describedbelow with the focus on such difference. Here the printer 200 is themulti-valued printer that is capable of varying the size of dots. Thefollowing description is, however, similarly applicable to themulti-valued printer that is capable of varying the density of inkinstead of the dot size and to the multi-valued printer that is capableof varying both the dot size and the density of ink.

Like the image printing routine of the first embodiment, the imageprinting routine of the third embodiment reads object image data to beconverted (step S600) and makes the input image data subjected to colorconversion (step S602). The color conversion process converts the inputimage data into tone data expressed by the tone values of the respectivecolors C, M, Y, and K.

The printer 200 of the first embodiment discussed above is not capableof varying the dot size but takes only either of the two states, thedot-on state or the dot-off state, with regard to each color. The imageprinting process of the first embodiment thus determines the dot on-offstate in the individual pixels immediately after the color conversion.The printer 200 of the third embodiment, however, is capable of varyingthe dot size and creating three different size dots, that is, thelarge-size dot, the medium-size dot, and the small-size dot. The imageprinting process of the third embodiment thus converts the tone dataobtained by the color conversion into large-size dot data, medium-sizedot data, and small-size dot data with regard to each color (step S604).

A conversion table shown in FIG. 21 is used for conversion of the tonedata into the large-size dot data, the medium-size dot data, and thesmall-size dot data. The conversion table stores variations inlarge-size dot data, medium-size dot data, and small-size dot dataagainst the tone data. The tone data after color conversion is furtherconverted by referring to this conversion table.

The large-size dot data, the medium-size dot data, and the small-sizedot data respectively go through a resolution conversion process (stepS606). Any of diverse methods is applicable to the resolution conversionprocess. For the simplicity of explanation, the technique of dividingoriginal pixels into smaller pixels is adopted for the resolutionconversion like the first embodiment. The tone value allocated to eachoriginal pixel is set to all the smaller pixels as divisions of theoriginal pixel.

After conversion of the input resolution into the printing resolution,the resolution-converted data goes through a number data generationprocess (step S608). In the structure of the third embodiment, theprinter 200 is capable of creating the three different size dots, thatis, the large-size dot, the medium-size dot, and the small-size dot. Thenumber data generation process of the third embodiment thus generatesdot number data with regard to the three different size dots and outputsthe generated dot number data to the printer 200.

FIG. 22 shows a process of generating dot number data from dot dataaccording to the number data generation process of the third embodiment.FIG. 22( a) shows settings of dot data with regard to the large-size,medium-size, and small-size dots to the individual pixels included ineach pixel group. Each solid rectangle represents one pixel group. Eachpixel group includes a predetermined number of multiple pixels, and dotdata are actually set to each of the multiple pixels. For the graphicsimplicity, however, the individual pixels are omitted from theillustration, and the dot data are set to each pixel group in theillustration. For example, the expression of Data(L,M,S)=(2,90,32) in apixel group on the upper left corner of FIG. 22( a) means that thelarge-size dot data ‘2’, the medium-size dot data ‘90’, and thesmall-size dot data ‘32’ are set to the individual pixels included inthis pixel group. As mentioned above in relation to the firstembodiment, when all the pixels included in each pixel group have anidentical tone value, the resolution conversion process may be omittedand the number data generation process may take charge of thesubstantial resolution conversion.

The number data generation process of the third embodiment processes thedot data with regard to the respective size dots and generates dotnumber data as shown in FIG. 22( b). As in the representation of FIG.22( a), each solid rectangle in FIG. 22( b) represents a pixel group,which includes a predetermined number of multiple pixels. The individualpixels are omitted from the illustration, and the dot number data aregenerated with respect to each pixel group. For example, the expressionof Dot(L,M,S)=(1,2,1) in the pixel group on the upper left corner ofFIG. 22( b) means that the dot number data ‘1’, ‘2’, and ‘1’ aregenerated respectively as the dot numbers of the large-size dot, themedium-size dot, and the small-size dot to be created in this pixelgroup. The details of the number data generation process of the thirdembodiment will be discussed later.

The printer 200 receives the dot number data output from the computer100 and carries out a pixel position determination process (step S610).Since the structure of the third embodiment creates the three differentsize dots, that is, the large-size dot, the medium-size dot, and thesmall-size dot, the pixel position determination process determines thepositions of dot-on pixels with regard to these three different sizedots. The details of the pixel position determination process of thethird embodiment will be discussed later.

After determination of the pixel positions for dot formation, theprinter 200 repeats the main scans and the sub-scans of the carriage 240and drives the ink ejection heads to create the large-size dots, themedium-size dots, and the small-size dots on the printing paper (stepS612). A resulting image corresponding to image data is thus printed onthe printing paper.

D-2. Number Data Generation Process in Third Embodiment

The following describes the details of the number data generationprocess of the third embodiment. FIG. 23 is a flowchart showing thedetails of the number data generation process in the third embodimentexecuted by the CPU of the computer 100.

When the number data generation process of the third embodiment starts,the CPU first collects a predetermined number of multiple pixels in theinput image data to one pixel group (step S700). The resolutionconversion process (step S606 in the flowchart of FIG. 20) prior to thenumber data generation process has divided each original pixel intosmaller pixels, as in the procedure of the first embodiment. Multiplesmaller pixels as divisions of one identical pixel are thus gathered toone pixel group.

The process then reads the large-size dot data, the medium-size dotdata, and the small-size dot data with respect to the individual pixelsin the pixel group (step S702). The individual pixels included in thepixel group are divisions of an identical original pixel and have anidentical tone value. The process may thus read the dot data of only onepixel in the pixel group, instead of reading the dot data of all theindividual pixels in the pixel group.

After reading the dot data regarding the respective size dots, theprocess refers to a dither matrix to determine the dot on-off state withrespect to the large-size dot, the medium-size dot, and the small-sizedot (step S704). FIG. 24 shows a method of determining the dot on-offstate of the large-size dot, the medium-size dot, and the small-size dotin each target pixel by referring to the dither matrix. In theillustrated example of FIG. 24, the dot data to be processed and thedither matrix referred to are respectively those shown in FIG. 22( a)and shown in FIG. 8.

FIG. 24 shows a process of determining the dot on-off state of the threedifferent size dots in a pixel group on the upper left corner of animage. Each rectangle of the thick solid line represents one pixelgroup, and the divisions of the pixel group by the thin broken linesrepresent multiple pixels constituting the pixel group. The numeralshown in each pixel is a threshold value set at the correspondingposition in the dither matrix.

The method of determining the dot on-off state first compares thelarge-size dot data with the threshold values set at the correspondingpositions in the dither matrix. When the large-size dot data is greaterthan the corresponding threshold value in a target pixel, a large-sizedot is to be created in the target pixel. FIG. 24( a) showsdetermination of the large-size dot on-off state with regard to theindividual pixels in the pixel group. The large-size dot data is ‘2’ inthis pixel group. Only in the upper left pixel having the correspondingthreshold value ‘1’ of the dither matrix, the large-size dot data ‘2’ isgreater than the threshold value. The corresponding threshold values ofthe dither matrix are greater than the large-size dot data ‘2’ in allthe other pixels. Namely only one large-size dot is to be created inthis pixel group. The pixel having the threshold value ‘1’ of the dithermatrix is filled with dense slant lines in FIG. 24( a). This expressesexpected formation of a large-size dot in this pixel.

On completion of determination of the large-size dot on-off state, themethod subsequently determines the medium-size dot on-off state. Themethod adds the medium-size dot data to the large-size dot data togenerate intermediate data of the medium-size dot and compares theintermediate data of the medium-size dot with the threshold values setat the corresponding positions in the dither matrix. When theintermediate data of the medium-size dot is greater than thecorresponding threshold value in a target pixel, a medium-size dot is tobe created in the target pixel. The pixel specified for expectedformation of a large-size dot is omitted from the object ofdetermination of the medium-size dot on-off state. In the concreteexample of FIG. 24( b), summation of the large-size dot data ‘2’ and themedium-size dot data ‘90’ gives the intermediate data ‘92’ of themedium-size dot. This intermediate data ‘92’ of the medium-size dot iscompared with the corresponding threshold values in the dither matrix.The upper left pixel in the pixel group has been determined as thelarge-size dot-on pixel and is thus omitted from the object ofcomparison. In the pixels having the corresponding threshold values ‘42’and ‘58’ of the dither matrix, the intermediate data ‘92’ of themedium-size dot is greater than the respective threshold values.Medium-size dots are thus to be created in these pixels. These pixelsare filled with slant lines in FIG. 24( b). This expresses expectedformation of medium-size dots in these pixels.

On completion of determination of the medium-size dot on-off state, themethod determines the small-size dot on-off state. The method adds thesmall-size dot data to the intermediate data of the medium-size dot togenerate intermediate data of the small-size dot and compares theintermediate data of the small-size dot with the corresponding thresholdvalues of the dither matrix with respect to the remaining pixels thathave not been specified as dot-on pixels. When the intermediate data ofthe small-size dot is greater than the corresponding threshold value ina target pixel, a small-size dot is to be created in the target pixel.In the concrete example of FIG. 24( c), summation of the intermediatedata ‘92’ of the medium-size dot and the small-size dot data ‘32’ givesthe intermediate data ‘124’ of the small-size dot. This intermediatedata ‘124’ of the small-size dot is compared with the correspondingthreshold values in the dither matrix. In the pixel having thecorresponding threshold value ‘109’ of the dither matrix, theintermediate data ‘124’ of the small-size dot is greater than thethreshold value. A small-size dot is thus to be created in this pixel,which is filled with sparse slant lines in FIG. 24( c). This expressesexpected formation of a small-size dot in this pixel. The processdetermines the dot on-off state of the large-size dot, the medium-sizedot, and the small-size dot with respect to the individual pixels in thepixel group in the above manner at step S704 in the flowchart of FIG.23.

After determination of the dot on-off state with regard to therespective size dots, the process stores the numbers of the respectivesize dots to be created in the pixel group (step S706). In theillustrated example of FIG. 24, one large-size dot, two medium-sizedots, and one small-size dot are stored as the numbers of the respectivesize dots to be created in the pixel group.

After storage of the numbers of the respective size dots to be createdin the pixel group having the predetermined number of multiple pixels,the process determines whether the processing has been completed withregard to all the pixels included in the input image data (step S708).When there is any unprocessed pixel (step S708: No), the process returnsto step S700 to set a next pixel group and repeats the subsequent seriesof processing. When the processing has been completed with regard to allthe pixels included in the input image data (step S708: Yes), theprocess outputs the numbers of the respective size dots stored in unitsof pixel groups to the printer 200 (step S710). For further reduction ofthe data volume output to the printer 200, the dot number data of therespective size dots are encoded as discussed below.

FIG. 25 conceptually shows a conversion table referred to for encodingthe dot number data of the respective size dots. The conversion tablestores settings of code numbers allocated to respective combinations ofthe numbers of the large-size dot, the medium-size dot, and thesmall-size dot. For example, a code number ‘0’ is allocated to acombination of the numbers of the large-size dot, the medium-size dot,and the small-size dot all equal to 0. A code number ‘1’ is allocated toa combination of the number of the small-size dot equal to 1 and thenumbers of the large-size dot and the medium-size dot both equal to 0.

Output of the code number representing the numbers of the respectivesize dots to the printer 200 desirably reduces the output data volume.The reason of data volume reduction is described briefly. Each pixelgroup consists of 8 pixels, and the maximum number of each size dot isaccordingly 8. The dot number data of each size dot accordingly requiresa 4-bit data volume. There are three different size dots, so that eachpixel group requires a 12-bit data volume. The code numbers range from 0to 164 as shown in FIG. 25. Each pixel group accordingly requires onlyan 8-bit data volume for the encoded data. Such encoding thus readilyreduces the required data volume to ⅔.

The combinations of the numbers of the respective size dots include manyunpractical combinations for actual printing. Allocation of code numbersto these unpractical combinations is naturally not required. Allocationof the code numbers to only practical combinations further reduces therequired data volume of each pixel group to be less than 8 bits.

The process encodes the dot number data of the respective size dots andoutputs the encoded data to the printer 200 at step S710 in theflowchart of FIG. 23, on this account. After output of the encoded dotnumber data of the respective size dots in units of pixel groups to theprinter 200, the program terminates the number data generation processof the third embodiment and returns to the image printing routine ofFIG. 20.

The number data generation process of the third embodiment discussedabove determines the dot on-off state of the respective size dots withspecification of dot-on pixel positions. The information onspecification of dot-on pixel positions is, however, omitted from thetransferred data, and only the information on the numbers of therespective size dots to be created in each pixel group is transferred tothe printer 200. In view of such omission, the number data generationprocess may determine the numbers of the respective size dots withoutspecification of dot-on pixel positions, like the first modified exampleof the first embodiment described above (see FIG. 13). FIG. 26conceptually shows a process of determining the numbers of therespective size dots without specification of dot-on pixel positions.

The process of FIG. 26 determines the numbers of the respective sizedots without specification of the dot-on pixel positions in the pixelgroup of FIG. 24. A simplified dither matrix is applicable to determinethe numbers of multiple different size dots, as in the case ofdetermination of the number of a single size dot. As describedpreviously, while the standard dither matrix stores the threshold valuesone-to-one mapped to the respective pixel positions, the simplifieddither matrix stores the threshold values that are not one-to-one mappedto the respective pixel positions but are collectively mapped to therespective pixel groups. Namely a set of multiple threshold values isone-to-one mapped to one pixel group in the simplified dither matrix.For example, the threshold values in the dither matrix are one-to-onemapped to the respective pixels in the pixel group shown in FIG. 24. Inthe simplified dither matrix of FIG. 26, on the other hand, a set ofthreshold values {255,212,177,170,109,58,42,1} is one-to-one mapped tothe pixel group. The following describes the method of determining thenumbers of the respective size dots without specification of dot-onpixel positions according to the example of FIG. 26.

This modified method sequentially determines the number of thelarge-size dot, the number of the medium-size dot, and the number of thesmall-size dot in this order. FIG. 26( a) shows determination of thenumber of the large-size dot, FIG. 26( b) shows determination of thenumber of the medium-size dot, and FIG. 26( c) shows determination ofthe number of the small-size dot. The method first compares thelarge-size dot data with a set of threshold values mapped to a pixelgroup. The number of the threshold values smaller than the large-sizedot data is given as the number of the large-size dot to be created inthis pixel group. In the illustrated example of FIG. 26( a), thelarge-size dot data is ‘2’, and there is only one threshold valuesmaller than this large-size dot data ‘2’. The number of the large-sizedot to be created in this pixel group is accordingly 1. The thresholdvalue ‘1’ is covered with dense slant line in FIG. 26( a). Thisexpresses expected formation of a large-size dot for this thresholdvalue.

After determination of the number of the large-size dot, the method addsthe medium-size dot data to the large-size dot data to generateintermediate data of the medium-size dot and compares the intermediatedata of the medium-size dot with the set of threshold values. The numberof the threshold values smaller than the intermediate data of themedium-size dot is given as the number of the medium-size dot to becreated in this pixel group. The threshold value set for formation ofthe large-size dot is omitted from the object of such comparison. In theillustrated example of FIG. 26( b), summation of the large-size dot data‘2’ and the medium-size dot data ‘90’ gives the intermediate data ‘92’of the medium-size dot. Except the threshold value ‘1’ set for formationof the large-size dot, there are two threshold values smaller than theintermediate data ‘92’ of the medium-size dot. The number of themedium-size dot to be created in this pixel group is accordingly 2. Thethreshold values ‘42’ and ‘58’ are covered with slant lines in FIG. 26(b). This expresses expected formation of medium-size dots for thesethreshold values.

After determination of the number of the medium-size dot, the methodadds the small-size dot data to the intermediate data of the medium-sizedot to generate intermediate data of the small-size dot and compares theintermediate data of the small-size dot with the set of threshold valuesto determine the number of the small-size dot to be created in the pixelgroup. In the illustrated example of FIG. 26( c), summation of theintermediate data ‘92’ of the medium-size dot and the small-size dotdata ‘32’ gives the intermediate data ‘124’ of the small-size dot. Amongthe remaining threshold values that have not been set for dot formation,there is only one threshold value smaller than the intermediate data‘124’ of the small-size dot. The number of the small-size dot to becreated in this pixel group is accordingly 1. The threshold value ‘109’is covered with sparse slant lines in FIG. 26( c). This expressesexpected formation of a small-size dot for this threshold value.

This modified method refers to the simplified dither matrix anddetermines the numbers of the respective size dots without specificationof dot-on pixel positions. The numbers of the respective size dots aredetermined by simple comparison of the dot data or the intermediate datawith the set of threshold values. This method does not requirecomparison between the dot data with the individual threshold valuesallocated to the respective pixels. This ensures quicker generation ofthe dot number data with regard to the respective size dots.

D-3. Pixel Position Determination Process of Third Embodiment

As described above, the number data generation process of the thirdembodiment generates the dot number data representing the numbers of therespective size dots to be created in each pixel group and transfers thedot number data to the printer 200. The printer 200 receives thetransferred data, determines the positions of the dot-on pixels of therespective size dots, and creates the respective size dots to print aresult image on the printing paper. The following describes theprocedure of determining the positions of the dot-on pixels of therespective size dots according to the dot number data.

FIG. 27 is a flowchart showing the details of the pixel positiondetermination process in the third embodiment executed by the CPU of thecontrol circuit 260 in the printer 200.

The pixel position determination process of the third embodiment firstselects a target pixel group as an object of pixel positiondetermination (step S800) and extracts data of the selected target pixelgroup from the transferred data (step S802). The dot number data havebeen encoded prior to transfer as mentioned previously in relation toFIG. 25. The process accordingly decodes the encoded data to the dotnumber data representing the numbers of the respective size dots to becreated in the target pixel group (step S804).

The method refers to the conversion table shown in FIG. 25 and obtainsthe decoded data corresponding to the code number. For example, when theencoded data is ‘162’, the dot numbers corresponding to the code number‘162’ are read from the conversion table. Namely the encoded data ‘162’is decoded to the dot number data representing seven large-size dots,zero medium-size dot, and one small-size dot.

After obtaining the decoded dot number data representing the numbers ofthe respective size dots, the process refers to a dither matrix todetermine the positions of the dot-on pixels of the respective size dots(step S806). The concrete procedure of determination is discussed belowwith reference to FIG. 28.

FIG. 28 shows a process of determining the positions of the dot-onpixels of the respective size dots according to given dot number data ofthe respective size dots in one pixel group by referring to the dithermatrix. Each rectangle of the thick solid line represents one pixelgroup, and the divisions of the pixel group by the thin broken linesrepresent multiple pixels constituting the pixel group. The numeralshown in each pixel is a threshold value set at the correspondingposition in the dither matrix. The dither matrix used here is identicalwith the dither matrix used for determination of the numbers of therespective size dots.

In this illustrated example, it is assumed that one large-size dot, twomedium-size dots, and one small-size dot are to be created in the pixelgroup. The method first determines the positions of the large-sizedot-on pixels. As mentioned previously, the threshold values in thedither matrix show the potential for dot formation. One large-size dotis thus to be created in the pixel having the smallest threshold value.The method determines the positions of the medium-size dot-on pixelsafter determination of the positions of the large-size dot-on pixels.Two medium-size dots are to be created, while the pixel having thesmallest threshold value has already been set for formation of thelarge-size dot. Two medium-size dots are thus to be created in thepixels having the second smallest threshold value and the third smallestthreshold value. The method determines the positions of the small-sizedot-on pixels after determination of the positions of the medium-sizedot-on pixels. One small-size dot is to be created, while the pixelshaving the smallest, the second smallest, and the third smallestthreshold values have already been set for formation of the large-sizedot and the medium-size dot. One small-size dot is thus to be created inthe pixel having the fourth smallest threshold value.

In the illustrated example of FIG. 28, the positions of the dot-onpixels are determined in the order of the large-size dot, themedium-size dot, and the small-size dot. The pixel filled with denseslant lines denotes the pixel set for formation of a large-size dot. Thepixels filled with slant lines denote the pixels set for formation ofmedium-size dots. The pixel filled with sparse slant lines denotes thepixel set for formation of a small-size dot. The process refers to thedither matrix and determines the positions of the dot-on pixels withregard to the respective size dots at step S806 in the flowchart of FIG.27.

After determination of the positions of the dot-on pixels with regard tothe respective size dots in one pixel group, the process determineswhether the pixel position determination has been completed for the dataof all the pixel groups received from the computer 100 (step S808). Whenthere is any unprocessed pixel group (step S808: No), the processreturns to step S800 to set a next pixel group and repeats thesubsequent series of processing. When the dot-on pixel positions havebeen determined for all the pixel groups (step S808: Yes), the programterminates the pixel position determination process of FIG. 27 and goesback to the image printing routine. The printer 200 then creates therespective size dots on the printing paper to print a resultingprocessed image according to the image data.

The image printing process of the third embodiment discussed above iseffectively applied to the printer 200 as the multi-valued printer. Theprinter 200 receives the dot number data of the respective size dotstransferred from the computer 100 and prints a resulting processed imageaccording to the transferred data. This arrangement attains quicker datatransfer, compared with the method of transferring the dot state datarepresenting the dot on-off state of the individual pixels. Even when animage includes a large number of pixels, the procedure of the thirdembodiment thus ensures high-speed image printing.

E. Modifications

The technique of the invention is not restricted to the embodiments ortheir modified examples discussed above. There may be diversity of othermodifications. Some examples of possible modification are describedbriefly below.

(1) First Modified Example

The number data generation process in any of the above embodimentsapplies the dither method to determine the dot on-off state. Any othertechnique may alternatively be adopted for determination of the dotnumber to attain the adequate density according to the tone values ofthe image data.

For example, one modified example may adopt a number data generationprocess shown in FIG. 30 to calculate a mean tone value of respectivepixels included in each pixel group and unequivocally determine thenumbers of respective size dots to be created in the pixel groupaccording to the calculated mean tone value. The procedure of thismodified example is briefly described with reference to the flowchart ofFIG. 30.

The number data generation process of this modified example firstcollects a predetermined number of multiple pixels to one pixel group(step S900) and calculates a mean tone value of the respective pixelsincluded in the pixel group (step S902). The process then unequivocallydetermines the numbers of the respective size dots to be created in thepixel group according to the calculated mean tone value. A concreteprocedure sets variations in numbers of the respective size dots to becreated in one pixel group against the mean tone value as shown in themap of FIG. 31. The procedure reads the numbers of the respective sizedots corresponding to the given mean tone value of the pixel group fromthis map. The map of FIG. 31 shows the variations in numbers of thethree different size dots, but there may be a greater or less number ofvariable-size dots.

The process stores the determined numbers of the respective size dotswith respect to the pixel group (step S906) and determines whether theprocessing has been completed for all the pixels (step S908). When thereis any unprocessed pixel, the process returns to step S900 and repeatsthe subsequent series of processing. When the processing of all thepixels has been completed, on the other hand, the process encodes thestored numbers of the respective size dots according to the method ofFIG. 29 discussed above and outputs the encoded dot number data to theprinter 200 (step S910).

The printer 200 receives the dot number data output from the computer100 and executes the pixel position determination process discussedabove to print a resulting processed image. The procedure of thismodified example readily generates the dot number data with regard toeach pixel group. This arrangement ensures quick output of the dotnumber data and thereby high-speed image printing.

(2) Second Modified Example

In any of the above embodiments, the number data generation processfirst collects a predetermined number of multiple pixels to one pixelgroup and then determines the number of dots to be created in the pixelgroup. One modified procedure may first determine the dot on-off stateof individual pixels and then collect every predetermined number ofmultiple pixels to one pixel group.

For example, a modified process shown in the flowchart of FIG. 32applies an error diffusion method to determine the dot on-off state ofall the pixels included in the input image data (step S950). Themodified process then collects a predetermined number of multiple pixelsto one pixel group (step S952), and counts and stores the number of dotsto be created in the pixel group (step S954).

After storage of the dot number data with regard to one pixel group, theprocess determines whether the processing has been completed for all thepixels (step S956). When there is any unprocessed pixel, the processgoes back to step S900 and repeats the above series of processing. Whenthe processing of all the pixels has been completed, on the other hand,the process outputs the dot number data stored in units of pixel groupsto the printer 200 (step S958).

The printer 200 receives the dot number data output from the computer100 and executes the pixel position determination process describedabove to determine the positions of the dot-on pixels in each pixelgroup. The modified procedure of FIG. 32 adopts the error diffusionmethod for determination of the number of dots to be created in eachpixel group. This procedure may refer to the dither matrix to determinethe positions of the dot-on pixels.

As is known in the art, application of the error diffusion method todetermine the dot on-off state gives the substantially noise-free, goodpicture quality in an area of the low dot formation density. The errordiffusion method is especially effective for printing an image of thelow dot formation density, for example, where pixel groups with dotformation are sparsely distributed among a large number of pixels groupswith no dot formation. Determination of the number of dots to be createdin each pixel group according to the error diffusion method enables thedots to be adequately distributed and gives a high-quality resultingimage.

(3) Third Modified Example

Any of the embodiments discussed above determines the positions of thedot-on pixels by referring to the dither matrix. The method of using thepriority order matrix for the pixel position determination does notdirectly refer to the dither matrix but indirectly uses the dithermatrix to determine the positions of the dot-on pixels, since thepriority order matrix is prepared corresponding to the dither matrix.Such dependence on the dither matrix is, however, not essential as longas the positions of the dot-on pixels are determinable with differentsettings of the priority order to the respective pixel groups.

For example, a modified process shown in the flowchart of FIG. 33 storesmultiple priority orders and selects an arbitrary priority order foreach pixel group among the stored multiple priority orders to determinethe positions of the dot-on pixels. The procedure of this modifiedexample is briefly described with reference to the flowchart of FIG. 33.

When the pixel position determination process of this modified examplestarts, the CPU of the control circuit 260 in the printer 200 firstselects a target pixel group as an object of the pixel positiondetermination and obtains the dot number data of the selected targetpixel group (step S970). The process then selects arbitrary one amongmultiple priority orders stored in advance (step S972). The multiplepriority orders of respective pixels in each pixel group have beenstored in the ROM of the control circuit 260. FIG. 34 shows multiplepriority orders stored in the ROM. The process selects one of themultiple priority orders at step S972.

The process refers to the selected priority order and determines thepositions of the dot-on pixels in the target pixel group (step S974).After determination of the positions of the dot-on pixels in one pixelgroup, the process determines whether the processing has been completedfor all the pixel groups (step S976). When there is any unprocessedpixel group, the process goes back to step S970 and repeats the aboveseries of processing to determine the positions of the dot-on pixels ina next target pixel group. This series of processing is repeated untilthe processing of all the pixel groups has been completed.

The procedure of this modified example determines the positions of thedot-on pixels according to the dot number data of each pixel group.Different priority orders are generally selected for the individualpixel groups to be referred to for determination of the positions of thedot-on pixels. This arrangement prevents dot formation in an identicalpattern and thus desirably avoids potential deterioration of the picturequality.

The embodiments and their modified examples discussed above are to beconsidered in all aspects as illustrative and not restrictive. There maybe many other modifications, changes, and alterations without departingfrom the scope or spirit of the main characteristics of the presentinvention. For example, the software programs (application programs)that attain the diverse functions discussed above may be supplied to themain memory of the computer system or to an external storage device viaa communication line and be executed by the computer system. Thesoftware programs may otherwise be read from CD-ROMs or flexible disksto be executed.

The embodiments and the modified examples discussed above regard theprinters that create dots to print an image on printing paper. Thetechnique of the invention is, however, not restricted to the printersbut is also effectively applicable to liquid crystal display devicesthat disperse luminescent spots at an adequate density on a liquidcrystal display screen to express an image of continuously varying tone.

1. An image output control system comprising an image processing device that makes image data subjected to a preset series of image processing and an image output device that creates dots according to a result of the preset series of image processing to output an image, said image processing device comprising: a pixel number increase module that processes each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image; a pixel group setting module that collects the multiple pixels generated from each original pixel of the image to each pixel group; a dot number specification module that causes image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifies number of dots to be created in each pixel group according to the representative image data; and a number data output module that outputs dot number data representing the specified number of dots with regard to each pixel group to said image output device; said image output device comprising: a number data receiving module that receives the output dot number data with regard to each pixel group; a priority order specification module that specifies a priority order of pixels for dot formation in each pixel group; a pixel position determination module that determines position of each dot-on pixel included in each pixel group, based on the received dot number data and the specified priority order; and a dot formation module that actually creates a dot at the determined position of each dot-on pixel.
 2. An image output control system in accordance with claim 1, wherein said priority order specification module selects one priority order for each pixel group, among multiple priority orders prepared in advance.
 3. An image output control system in accordance with claim 1, wherein said dot number specification module comprises: a mapping storage module that stores multiple mappings for conversion of the representative image data of each pixel group into the number of dots to be created in the pixel group; and a mapping selection module that selects one mapping for each pixel group among the stored multiple mappings; said dot number specification module specifying the number of dots to be created in each pixel group, based on the representative image data of the pixel group and the selected mapping.
 4. An image output control system in accordance with claim 3, wherein said mapping storage module stores multiple threshold value sequences, each consisting of plural threshold values corresponding to the predetermined number of plural pixels included in each pixel group, as the multiple mappings, said mapping selection module selects one threshold value sequence among the stored multiple threshold value sequences, and said dot number specification module sets number of smaller threshold values in the selected threshold value sequence that are smaller than the image data of each pixel group, to the number of dots to be created in the pixel group.
 5. An image output control system in accordance with claim 4, wherein said mapping storage module stores the plural threshold values of each threshold value sequence together with information on an order of magnitude of the respective threshold values in the threshold value sequence, and said dot number specification module refers to the order of magnitude and compares the image data of each pixel group with the plural threshold values of the selected threshold value sequence, so as to specify the number of dots to be created in the pixel group.
 6. An image output control system in accordance with claim 5, wherein said mapping storage module stores the plural threshold values of each threshold value sequence arranged in the order of magnitude as storage of the information on the order of magnitude.
 7. An image output control system in accordance with claim 5, wherein when the image data of one pixel group is greater than a preset first threshold value, said dot number specification module performs comparison with the image data of the pixel group in a descending order of the plural threshold values of the selected threshold value sequence, so as to specify the number of dots to be created in the pixel group.
 8. An image output control system in accordance with claim 5, wherein when the image data of one pixel group is smaller than a preset second threshold value, said dot number specification module performs comparison with the image data of the pixel group in an ascending order of the plural threshold values of the selected threshold value sequence, so as to specify the number of dots to be created in the pixel group.
 9. An image output control system in accordance with claim 5, wherein said dot number specification module start comparison between the image data of each pixel group and the plural threshold values of the selected threshold value sequence from a threshold value having a selected ordinal number corresponding to a most recently specified dot number, so as to specify the number of dots to be created in the pixel group.
 10. An image output control system in accordance with claim 4, wherein said mapping storage module stores a simplified dither matrix that includes the multiple threshold value sequences arranged in a preset two-dimensional array, as the multiple mappings, said mapping selection module selects one threshold value sequence corresponding to a position of each pixel group in the image, among the multiple threshold value sequences stored in the simplified dither matrix, and said dot number specification module specifies the number of dots to be created in each pixel group, based on comparison between the image data of the plural pixels included in the pixel group and the corresponding plural threshold values of the selected threshold value sequence.
 11. An image output control system in accordance with claim 10, wherein said priority order specification module comprises: a priority order storage module that stores a priority order matrix including the multiple priority orders of pixels for dot formation in each pixel group in a preset two-dimensional array, and the simplified dither matrix and the priority order matrix have an identical number of rows and an identical number of columns expressed by the number of pixels.
 12. An image output control system in accordance with claim 10, wherein said mapping storage module stores the simplified dither matrix that is generated by dividing a dither matrix, which maps threshold values to respective pixels arranged in a two-dimensional array, into multiple groups corresponding to multiple pixel groups and includes the multiple threshold value sequences arranged corresponding to the multiple groups, and said priority order specification module comprises: a priority order storage module that stores a priority order matrix that is generated by dividing the dither matrix into the multiple groups corresponding to the multiple pixel groups and includes the multiple priority orders arranged corresponding to the multiple groups, where the priority order is specified with regard to each pixel group based on a magnitude order of respective threshold values included in a corresponding group; and a priority order selection module that selects one priority order corresponding to a position of each pixel group in the image, among the multiple priority orders stored in the priority order matrix.
 13. An image output control system in accordance with claim 1, wherein said dot number specification module comprises: a dither matrix storage module that stores a dither matrix, which maps threshold values to respective pixels arranged in a two-dimensional array, said dot number specification module compares the representative image data of each pixel group with a threshold value stored at a corresponding position in the dither matrix, so as to specify the number of dots to be created in the pixel group, said priority order specification module selects a set of plural threshold values stored at positions in the dither matrix corresponding to respective pixels of each pixel group as the priority order specified for the pixel group, and said pixel position determination module determines the position of each dot-on pixel, based on the dot number data and the selected set of plural threshold values.
 14. An image processing device that causes input image data representing an image to go through a preset series of image processing and thereby generates control data, which is used for control of dot formation by an image output device that creates dots and outputs a resulting processed image, said image processing device comprising: a pixel number increase module that processes each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, a pixel group setting module that collects the multiple pixels generated from each original pixel of the image to each pixel group; a dot number specification module that causes image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifies number of dots to be created in each pixel group according to the representative image data; and a number data output module that outputs dot number data representing the specified number of dots with regard to each pixel group as the control data to said image output device.
 15. An image processing device in accordance with claim 14, wherein said dot number specification module comprises: a mapping storage module that stores multiple mappings for conversion of the representative image data of each pixel group into the number of dots to be created in the pixel group; and a mapping selection module that selects one mapping for each pixel group among the stored multiple mappings; said dot number specification module specifying the number of dots to be created in each pixel group, based on the representative image data of the pixel group and the selected mapping.
 16. An image output control method that makes image data subjected to a preset series of image processing and creates dots according to a result of the preset series of image processing to output an image, said image output control method comprising: pre-step of processing each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image; a first step of collecting the multiple pixels generated from each original pixel of the image to each pixel group; a second step of causing image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifying number of dots to be created in each pixel group according to the representative image data; a third step of specifying a priority order of pixels for dot formation in each pixel group; a fourth step of determining position of each dot-on pixel included in each pixel group, based on the specified number of dots and the specified priority order; and a fifth step of actually creating a dot at the determined position of each dot-on pixel.
 17. An image output control method in accordance with claim 16, wherein said second step comprises the steps of: storing multiple mappings for conversion of the representative image data of each pixel group into the number of dots to be created in the pixel group; and selecting one mapping for each pixel group among the stored multiple mappings; said second step specifying the number of dots to be created in each pixel group, based on the representative image data of the pixel group and the selected mapping.
 18. An image processing method that causes input image data representing an image to go through a preset series of image processing and thereby generates control data, which is used for control of dot formation by an image output device that creates dots and outputs a resulting processed image, said image processing method comprising the steps of processing each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, collecting the multiple pixels generated from each original pixel of the image to each pixel group; causing image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifying number of dots to be created in each pixel group according to the representative image data; and outputting dot number data representing the specified number of dots with regard to each pixel group as the control data to said image output device.
 19. An image output control program that is executed by a computer to make image data subjected to a preset series of image processing, create dots according to a result of the preset series of image processing, and thereby output an image, said image output control program causing the computer to attain: pre-function of processing each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, a first function of collecting the multiple pixels generated from each original pixel of the image to each pixel group; a second function of causing image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifying number of dots to be created in each pixel group according to the representative image data; a third function of specifying a priority order of pixels for dot formation in each pixel group; a fourth function of determining position of each dot-on pixel included in each pixel group, based on the specified number of dots and the specified priority order; and a fifth function of actually creating a dot at the determined position of each dot-on pixel.
 20. An image output control program in accordance with claim 19, wherein said second function comprises the functions of: storing multiple mappings for conversion of the representative image data of each pixel group into the number of dots to be created in the pixel group; and selecting one mapping for each pixel group among the stored multiple mappings, said second function specifying the number of dots to be created in each pixel group, based on the representative image data of the pixel group and the selected mapping.
 21. An image processing program that is executed by a computer to make image data of an image subjected to a preset series of image processing and thereby generate control data, which is used for control of dot formation by an image output device that creates dots and outputs a resulting processed image, said image processing program causing the computer to attain the functions of: processing each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, collecting the multiple pixels generated from each original pixel of the image to each pixel group; causing image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifying number of dots to be created in each pixel group according to the representative image data; and outputting dot number data representing the specified number of dots with regard to each pixel group as the control data to said image output device.
 22. An image output control system comprising an image processing device that makes image data subjected to a preset series of image processing and an image output device that creates dots according to a result of the preset series of image processing to output an image, said image processing device comprising: a generator that processes each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, a collector that collects the multiple pixels generated from each original pixel of the image to each pixel group; a number specification unit that causes image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifies number of dots to be created in each pixel group according to the representative image data; and a data transmitter that outputs dot number data representing the specified number of dots with regard to each pixel group to said image output device, said image output device comprising: a data receiver that receives the output dot number data with regard to each pixel group; a selector that selects a priority order of pixels for dot formation in each pixel group; an operator that determines position of each dot-on pixel included in each pixel group, based on the received dot number data and the selected priority order; and a dot formation unit that actually creates a dot at the determined position of each dot-on pixel.
 23. An image processing device that causes input image data representing an image to go through a preset series of image processing and thereby generates control data, which is used for control of dot formation by an image output device that creates dots and outputs a resulting processed image, said image processing device comprising: a generator that processes each original pixel of the image to generate multiple pixels having identical image data with image data of the original pixel, so as to increase a total number of pixels in the image, a collector that collects the multiple pixels generated from each original pixel of the image to each pixel group; a number specification unit that causes image data of respective pixels in each pixel group to be represented uniformly by preset representative image data and specifies number of dots to be created in each pixel group according to the representative image data; and a data transmitter that outputs dot number data representing the specified number of dots with regard to each pixel group as the control data to said image output device. 